I managed to get Openwrt installed to my DIR-882 using this method D-LINK Recovery Mode (DIR-882 DIR-878 DIR-867)
Same issue with openwrt 21.02 on (Linksys WRT1900ACS)
macbook pro from 2012 not at all. But with new wifi card from apple model macbook 2018 and huawei p20 and above should works just after reconnect the wifi.
Channel 36, 42, 48 ...etc testes the same.
hey guys i saw this post after my product return period was over, so i cant do anything. i bought a d link dir 2660 router for an offer price. i just verified that it have an official openwrt but didnt saw this known issue post. Is this issue still persist or is it fixed. i heard its not universal so i wish i wont be affected. if anyone know that someone is working on this please tell.
With the current r18812-918d4ab41e snapshot, I have yet to encounter the 5 GHz issue.
With that said,
I've only been running it for a few hours, so will post again if problem is encountered.
Is this common to all MT7621/MT7615N devices? The 5GHz connection on my Netgear R6700v2, which is also mt7621/mt7615 is rock solid in OpenWrt since I started with it using version 21.02.
Is it possible this is inherent in older driver versions? Because I do notice that 5GHz is very erratic on this device when DD-WRT is installed. Erratic as in speed reegotiations every second or two, usually from good 80+80 modes to very slow legacy modes and back again. And this even when signal is high and signal to noise has a 40+ dB spread.
Possibly.
Encountered 5 GHz problem with the DIR-878 and DIR-882 around the time of 21.02.1 stable release. Stable and snapshots around that time had the problem.
Stopped using the MediaTek routers for a few months and just started testing them again the last two days.
Update.
Noticed one of the really old notebooks does not connect on 5 GHz at all.
No problems with the 2.4 GHz.
Will test later on with a clean install of the operating system on the notebook when I have the time.
I can confirm, there does appear to be an issue on my R6700v2 on firmware 21.02.1. I am using my device in client-bridge mode. Since it is the client, it is easy to monitor its link speed. I reverted to release 21.02.1 and noted wide and rapid fluctuations in link speed. Jumping from very slow legacy modes up to high-speed AC modes and back again on a 1-2 second cycle.
After upgrading back to the latest snapshot, the link was far more stable and remained at high-speed AC modes.
It was similar to what I experienced when running DD-WRT on this device.
Just did a quick test of the latest Stable 21.02.2 release.
The 5 GHz WiFi problems still present.
Reverted to testing/using current Snapshot release.
Will see if 5 GHz problems appears over the long term.
I had to resort to using Windows (7) and Firefox. Even Chrome in Windows (7) wouldn't work with the D-Link Recovery GUI.
Same thing for me too. What's your feedback with snapshot?
It's only been a few days with snapshot released on February 25th but so far, no issues with 5 GHz WiFi.
With that said,
iPad would not detect a Brother AirPrinter.
An Archer C2600 with older stable release had no problem detecting the AirPrinter.
Update
With the recent SNAPSHOT r19100-cca5367f27, the 5GHz WiFi bug is definitely noticeable again. 
It does not display Connected, no internet.
but
Still fails to navigate/connect to any websites.
Currently testing snapshot r19129-e17c6ee627 release,
The 5 GHz WiFi problem has not been encountered for the last few hours...
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Update.
After 3 days, 5 GHz failure to navigate/connect to any websites has returned. 
Have you tried to isolate this issue?
Disabled any kind of hw acceleration / offload?
Does barebones install show the same issue?
Does all wireless connect to 5Ghz show the same issue or are you having trouble with a specific one? Have you tried to only connect one device at a time to see if there's specific one that causes the issue?
If WPA3 is enabled, have you tried to only use WPA2?
Does wpad-openssl show the same issues?
WPA2 used/tested.
Originally tested with an Archer C2600 running older OpenWrt stable release as comparison/reference. Same android phone and tablet used for comparison.
Note this topic was separated from original/another topic where number of members tried to troubleshoot the 5 GHz WiFi problem.
Just leaving updates on this forum as a warning/topic of caution of MT7621AT/MT7615N routers for others.
EDIT:
Decided to disable wireless entirely on one of the DIR-882 and use an EAP245 access point with tp-link's own firmware. Not a fan of tp-link brand but it was on sale and readily available at the time.
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I bought a RT-AC85P (without finding this thread before), installed OpenWrt 21.02.2 and now have a similar problem to the one described here.
Except:
- It affects both 2.4 and 5 GHz Wifi
- I only have this issue with my phone, a Samsung Galaxy A40 (SM-A405FN)
There are a number of other phones and laptops using the same Wifi and none of them have the same problem. Workarounds like setting Fragmentation + RTS/CTS or "Disable inactivity polling" unfortunately don't help 
I'll try a snapshot version next, my hope is that a new Linux kernel already has a fix for this.
Edit: Latest snapshot didn't fix it at all. I did find the relevant GitHub issue though: https://github.com/openwrt/mt76/issues/518
Thanks sfan5
That topic may be of interest if it leads to a fix.