4G LTE doesn't work on Cudy LT500

I have an OpenWrt based 4G LTE router - Cudy LT500.
Recently I encountered an issue with the SIM card that used to work on that device but doesn't anymore. Looking at the router UI (there is custom LuCi), I can see that it can connect to the 4G network but the connection quickly dies and the clients can't interact with it (not even a single ping goes through).
The built in troubleshooting mentions abnormal DNS.
I downloaded the system logs and there are some messages that look weird to me but I have no idea how to interpret them.
The APN settings are correct, as outlined by the ISP.
Could you please look through that file and give me some tips?
The log file is here

Your router is not supported in the current version of OpenWrt.
I suggest you to direct all the questions to the device manufacturer.

I am aware of that. It runs OpenWrt under the hoods though. This is why I am asking here.

It runs old and heavily customized version, that's why I mentioned in the current version.