So, the device includes a sim that has internet on it. There is a Mikrotik router with the primary internet on the ethernet 1 port, the Teltonik device is on the ethernet 7 port. When the primary internet is gone, the Mikrotik device switches to the Teltonik device. When there is internet from the Teltonik, the internal network cannot be accessed remotely. This network would be the VPN server but in this case it cannot be accessed from the outside. You can't even ping from an external network. Is the teltonik device currently in Bridge mode, should it be in Passtrough mode or is some firewall rule causing it?
If u need i can send printscreen or Ip or whatever u want.