I can set it to up to 27dBm in Luci, however this doesn't get applied. What does the wireless settings page of Luci show you? This is what I am getting, even though tx-power is set to 27. Notice the low value of 16 for Tx-power 
[quote]Mode: Master | SSID: Rejaap
BSSID: AA:BB:CC:DD:EE:FF | Encryption: WPA2 PSK (CCMP)
Channel: 100 (5.500 GHz) | Tx-Power: 16 dBm
Signal: 0 dBm | Noise: 0 dBm
Bitrate: 0.0 Mbit/s | Country: NL[/quote]
Compiled now from 17.01 branch and I lost 1 dBm 
Using 2 different devices: one based on MT7621A with MT7603 and MT7612 on PCIe and the other on MT7628A, that MT7603 on soc. and MT7612 on PCIe. Using the same driver and basically the same hardware from a wifi point of view I get completely different outputs
the MT7621 combo will give me 15 dBm on Channel 36 (county US)
the MT7628 combo will only do 7 dBm on the same channel and country settings.
both compiled with the same local build setup so all the git pull / feed updates etc. should be the same. I think we should create a new specific MT76 driver topic to compare notes on different hardware using the same driver (and getting similar Kernel panics).
Another observation I made: iw phy0 info will indicate 0 antennas for TX / RX??
Lede 17.01.2 Another one version with poor wifi for this router, i think its worthless to try to upgrade it anymore, wifi AC, 2 meters away and 650Mbps connection (on 866Mbps adapter) just a poor 4MB/sec Bandwidth, my old 941nd v3 on 2.4Ghz and 100Lan had better results almost full 11.2MB/sec (for its 100LAN)! D-link`s factory firmware is giving about 40MB/sec.Too bad.
I surrendered and after months of attempts I bought a wrt1200ac, it is another world ...
PS Try also latest snapshots
https://git.lede-project.org/?p=source.git;a=commit;h=77a64e8bffb7e7cb1ce46424fd188e2b997433fc
i am going to try it out, thank you very much! hope i will be lucky this time!
Remember that snapshots versions doesn't have LuCI (web interface) 
Does anyone know an update on this issue?
I did some tests today trying to solve some WiFi issues and found that I simply have the same problem on an 860L on the 5GHz radio.
With LEDE 17.01.2 "N" on 5 GHz seems acceptable (but still kind of feels low ranged), but "AC" on 5 GHz is bad at all.
Tried the magic rollback to stock firmware, go back to LEDE, as described earlier in this post, but that did not improve my situation.
I am sadly back on stock firmware now. WiFi throughput is fine now, but the situation is sad: Stock FW only has 5 wifi channels in 5 GHz (LEDE 20!), it's a 2 year old firmware and I miss my custom packages 
According to https://forum.openwrt.org/viewtopic.php?id=64333, DD-WRT seems to not be affected by that issue, so I might try that, yet dd-WRT does not seem to have custom packages.
.
try this Optimized build for the D-Link DIR-860L for me solved everything except 2.4Ghz disconnection issue of lede
Thanks. I'll come over to the other thread. Seems more reasonable to continue there.
In case someone else stumbles into this thread:
I applied the LEDE snapshot developer release from 2017-08-26.
This seemed to have to solve the issues, at least for the short time of 2h that its running now.
I had some minor issues in the first 10 minutes though: the 5GHz wifi would not come up at all when being started, only the 2.4 wifi. Several reboots with mostly random WiFi config changes later it would suddenly work flawlessly.
So when experiencing the same lagging 860L wifi: either wait for the successor of 17.01.2 or use a nightly snapshot for the time being.
I do not experience problems on 2.4 WiFi (I'll post in the other thread, in case someone is interested into following that)
@alexmow It is normal for the 5ghz wifi to take several minutes to come up on the DFS channels. Those few minutes are used for scanning for interfering radar equipment. If it detects radar equipment, it will automatically fall back to a non-DFS channel (probably 36). But if it doesn't detect any radar equipment during those minutes of listening, it will then come up on the DFS channel that was selected.
oh 
, thanks.
Is there actually a way to tell if LEDE is currently busy checking for radar?
Hi, i have solved 2.4 GHz performance problem by changing the poor antennas integrated with these:
https://www.aliexpress.com/item/1pcs-Dual-Band-Antenna-Laptop-Wifi-Internal-Antenna-8dbi-for-Pcb-Network-Card/32788264214.html?spm=a2g0s.9042311.0.0.1e824c4dJtOaJr
Very interesting! How did you hook them up to the DIR-860L? Also, do you happen to have benchmarks of the current performance? And maybe even benchmarks of the performance before the new antennas? That would allow for a direct apples to apples comparison 
First of all, I no longer have disconnections and speed is always constant with many connected clients.
It looks like another router!
Installation is very simple just use a drill with tip for wood 6.5 mm
Here I'm testing DIR-825 C1 dual band antennas.
As said if you do not want to hole these are better but I did not test them
https://it.aliexpress.com/item/1pcs-Dual-Band-Antenna-Laptop-Wifi-Internal-Antenna-8dbi-for-Pcb-Network-Card/32788264214.html
Interesting. Did the speeds increase as well?
Very much and now the transfer is always constant!
The problem with this router is the low quality integrated original antennas.
Is you have problems read also this:
Has anyone tried latest lede 17.01.3 ? have the issues with 5Ghz fixed? do we have a normal router now or keep the factory FW?
I have no problems with 5 GHz and is also stable with 2.4 GHz