I got mine fixed with help from the Teltonika support. From kernel logs they saw messages like:
kern.err kernel: [ 296.348569] mt76_wmac 10300000.wmac: MCU message 08 (seq 3) timed out
which means that wireless driver is timing out. Driver can be reset with command:
echo 1 > /sys/kernel/debug/ieee80211/phy0/mt76/full_reset
This however did not fix the issue until I did soft reboot for the device. Result is that I was now able to fix this completely from the remote and not doing power cycle at the site.
There’s also package “Wireless Reboot” which will reset wireless driver when timeouts occur. I hva not tested it yet.