Same here. I thought it could be Asus firmware issue from where we are coming, as earlier it would work with zysmon builds. So I reverted to multiple asus builds but she issue. Will test further over weekend. Also not sure how to copy factory factory1 partition that chunkee asked. WIP.
Check from drivers issue every install.
dmesg | grep ath10k
If you get failures, your issue probably will persist.
I had a similar problem when upgrading from the old snapshot to the release of OpenWrt. All problems were solved after installing OpenWrt via COM port. I'm sure you will also have problems with installing OpenWrt via COM port. I wrote about this earlier in this topic.
You can write the output of these commands before installing the stock firmware:
uname -r
cat /usr/lib/opkg/status|grep -E "Package|Version"|awk 'NR%2{printf "%s, ",$0;next;}1'|grep kernel
You can try another option to restore the router before returning to the stock firmware, my router did not work and this method did not help me, but maybe you can do it this way.
Use this method only if you already have OpenWrt installed and the router needs to be restored, this method does not delete the ubi0 partition, so this method can not be used to switch to OpemWrt from the stock firmware.
Preparation
Recovery via tftp
Return to stock firmware is also suitable for OpenWrt
http://lede-ac58u.zyxmon.org/HowToFlashENG.html (Restoring stock (factory) firmware)
I used this method, it works.
If you can not install OpenWrt in the above ways, install OpenWrt via the COM port.
I used this method (russian language) http://4pda.ru/forum/index.php?showtopic=790326&view=findpost&p=62828141 (open the spoiler of the Utility "Полезности" -> Via the serial port "Через последовательный порт")
The RT-AC58U.zip file can be downloaded here https://yadi.sk/d/JXW8CSQp3aYGJz
(I restored the router only in this way. No other recovery methods written previously did not restore the router.)
Can someone with a working openwrt setup on the ac58u step in and look into their configuration to help us out? I'm relatively new to the router hardware configuration so I have no clue why the wireless configuration is not working on my ac58u even after following flash instructions to the point.
I reset my device to defaults and extracted the contents of /etc/config/wireless:
# cat /etc/config/wireless
config wifi-device 'radio0'
option type 'mac80211'
option channel '11'
option hwmode '11g'
option path 'platform/soc/a000000.wifi'
option htmode 'HT20'
option disabled '1'
config wifi-iface 'default_radio0'
option device 'radio0'
option network 'lan'
option mode 'ap'
option ssid 'OpenWrt'
option encryption 'none'
config wifi-device 'radio1'
option type 'mac80211'
option channel '36'
option hwmode '11a'
option path 'platform/soc/a800000.wifi'
option htmode 'VHT80'
option disabled '1'
config wifi-iface 'default_radio1'
option device 'radio1'
option network 'lan'
option mode 'ap'
option ssid 'OpenWrt'
option encryption 'none'
Your openwrt is working? Can you let me know which version you're using as well as any specific installation instructions for your setup that I could follow? I'm being desperate because I've run out of options.
After going to 18.06.1 this is my output.
root@OpenWrt:~# uname -r
4.14.63
root@OpenWrt:~# cat /usr/lib/opkg/status|grep -E "Package|Version"|awk 'NR%2{printf "%s, ",$0;next;}1'|grep kernel
Package: kernel, Version: 4.14.63-1-55f569c94f99f14d0eaf9e39ac0ed4b4
root@OpenWrt:~#
Yes, it's working. I'm running a custom build compiled from source with minor modifications for my home infrastructure.
I installed OpenWRT using a serial/UART console (had to open the device), even before the device had official support. If I remember correctly, the procedure was:
When I have time again, probably the next weekend, I can flash a stock image and test the procedure with an official release (18.06.x) and add it to the wiki because there has been many people confused about the installation, and using modified versions of OpenWRT
The Factory and Factory2 partition data can be accessed via /dev/ubi0_1 and /dev/ubi0_2. You can ssh into the router and then do something like:
# cp /dev/ubi0_1 /tmp
# cp /dev/ubi0_2 /tmp
and optionally run sha256sum to check whenever they match or not:
# sha256sum /tmp/ubi0_[12]
b8744d39caf018573962b25ff6c28913776a1fa24495dcd55100ca2a1789ffaa /tmp/ubi0_1
b8744d39caf018573962b25ff6c28913776a1fa24495dcd55100ca2a1789ffaa /tmp/ubi0_2
(yours might be different)
As for getting the data from the router: you can use scp on your host PC to copy both files directly into your home directory.
(PC) $ scp root@OpenWrt:/tmp/ubi0_* ~
ubi0_1 100% 124KB 3.4MB/s 00:00
ubi0_2 100% 124KB 3.9MB/s 00:00
Or, you could connect a usb-stick to the router, etc.
Yes sir please do, the UART method is too scary for a newbie like me who wouldn't know how to put together an opened router except with glue! I'd keep an eye out for any instructions anyone could lend me for a flash without breaking the router's warranty terms. 
I did copy the files and checked it's sha256 signature, and they were exactly matching as below.
ubi0_1 80ED3B25A31EC77AAE9073A0710449A2B4BFD454E2FAB4CE3B84CC5703C745B6
ubi0_2 80ED3B25A31EC77AAE9073A0710449A2B4BFD454E2FAB4CE3B84CC5703C745B6
I have shared the files @ this location on my gdrive
Your RT-AC58U's Factory and Factory2 partitions must have been wiped at some point. Note: The ASUS u-boot tries to recreate the partitions, but it can only do so much and just fills it with the default values without any calibration data. Do you have a off-site backup (as on your PC) of the Factory or Factory2 partitions around? Because they would become really handy right about now.
If you don't have the original caldata around 
. But you could still try this file factory1 file createrd with the calibration values from my early RT-AC58U unit:
0: Download this file and transfer it to the RT-AC58U's /tmp directory:
(I don't have your WIFI MACs, so I just set wifi0 to 70:4d:7b:11:aa:bb and wifi1 70:4d:7b:11:aa:cc)
# ubiupdatevol /dev/ubi0_1 /tmp/factory1
( # ubiupdatevol /dev/ubi0_2 /tmp/factory1 - not necessary, but you might as well)
# rm -i /lib/firmware/ath10k/pre-cal-ahb-a[08]00000.wifi.bin
# rmmod ath10k_pci
# modprobe ath10k_pci
(now, both wifis should be back)
Of course, it goes without saying that this is not really optimal. And using a other device's caldata can cause the range, throughput and/or reliability to drop. However, it's better than throwing the router away.
Okay so this works. I am extremely thankful. I was able to make this device work with asus stock all this time, but now lede also works. You said i need my own caldata. Looking further if there is a way to generate one. I'm sorry but somehow I can't sleep nicely if my router is not optimum.
If there is anything I need to do to cleanup, do let me know, like how do I change to my wifi mac id, etc. I'll dig further and would come back If I can find something else. Thanks.
Great you got the device working again.
I take it you didn't have backups of your factory partitions. Well, my unit is an early RT-AC58U model (back in 2016) for the EU region. I know that ASUS did some revisions on the boardfiles later on too, so I almost expect some stuff has changed. But as long as it's working now, there's not much you can do to improve it... (Maybe start a call for other people to upload their pre-cal-* files for this router so we could at least look where they changed something).
That would involve expensive software, tools and test jigs. In fact, it's much cheaper to just get another one.
Oh, there are several ways to change the MACs. Some are temporary (i.e.: last as long as the installation), while others are more involved.
But for any of those steps, you would need to reconstruct the WIFI0 and WIFI1 MAC first.
On my unit, there's a sticker with a MAC on the back. On the original ASUS FW, That MAC was used as the LAN and WIFI0 mac and the WAN and WIFI1 MACs were pretty +4 to that.
Once you have the MACs, you can decide how much time you want to invest.
Can you confirm if it's working well now? Any issues you're facing or is it all good now?
Okay so how do I proceed with these instructions from a stock asus firmware? I'm guessing I need to flash to the latest openwrt first and then transfer the file to the router? Also, what is the procedure to transfer the file from my pc to the router? Sorry for being a pleb.
It's working well. Now that @chunkeey said it may not be optimum, I'm imagining that may be ac WiFi may not be running at full speed, above 100Mbps. Don't have any reason to suspect though, and everything else is good. Finally back on Lede.
Follow the procedure @Sold mentioned above to get to open wrt 18.0.1. For transferring file use mobaxterm free on windows. It's has integrated file browser with, probably all kinds of connected protocol, including com. Also take backup of your ubi0 files.