Ernestas, thank you for getting back.
I didn't notice anything in particular that disrupts the connection.
WAN port of the RUT950 router is on a private LAN behind the NAT. A few devices are connected to the LAN port of the RUT950 router with practically close to zero http traffic. Connectivity to the ZeroTier network drops even with no activity on the connected devices. At all times the WAN port of the RUT950 router was pingable and there was no indication of any problems on the Status menu of RUT950.
Internet->Home router with NAT -> RUT950-WAN interface -> RUT950-LAN ->Ethernet switch -> three IP phones.
There is a GSM SIM card in the RUT950 router for fail-over, however there were no Internet interruptions with the wired Internet.
I guess you can re-create the problem using the same setup.
We have been experiencing the same issue with the RUT240 router.