i have a Xiaomi Mi-R3G(gigabyte) router, installed openwrt four years ago from source, it had issue - in lan the network would re-connect once 1-2 secs (dont know about WAN). it was impossible to use, i changed to another firmware, not openwrt
time passed, my lan upgraded to gigabyte cables, i compiled openwrt again, the re-connections were no longer an issue! it was ok
then i noticed these re-connections appeared suddenly in vmware in bridge mode.
then more: if i connect to router, i can even use unmanaged switch (mercusys ms108g) with 100Mbit cord, then the device starts having these re-connects again. gigabyte clients don't have this issue.
then i noticed that ethernet device it can be 100Mbit, but the cable is gigabyte - the issue also disappears. I thought it is a bug in drivers for a long time.
But today I found one firmware at github, a fork, https://github.com/coolsnowwolf/lede compiled it,installed. Connected device with 100Mbit cable, waited 40 mins, disconnects are missing. in vmware they're also absent.
Hope this can be fixed in openwrt
The problem is observed on versions: 18.06 - present (21.02), and nightly.
I'm ready to be a tester, provide any logs, and flashing the router in testing the fix.