Modem connection issues on ZBT-WE826-T2

Hello! I’ve bought a new router ZBT-WE826-T2, however after starting up that’s happening: modem starting up, after couple of minutes error, disconnecting the modem interface. What should I do? I can provide logs if needed, however I’m kind of newbie in such things. Thank you in advance for helping!

Please confirm you're using the official and up-to-date OpenWrt version.

Powered by LuCI openwrt-21.02 branch (git-23.093.57360-e98243e) / OpenWrt 21.02.7 r16847-f8282da11e

This is no. If this is a vendor supplied software, you need to contact your router vendor.
Otherwise feel free to install the most recent official version of OpenWrt and ask questions here.

Unfortunately, vendor’s solution didn’t help, so I decided to ask here, thanks, I’ll try anything else

https://openwrt.org/toh/zbtlink/we-826

Hi.
As suggested by @frollic go to we826 openwrt toh.
I use a couple of we826 t2 with EC25 lte modem for 2 remote locations, and they are running openwrt both flawless for 2 years.