Hello,
It seems that dnsmasq service is crashing due to missing DNS server(s) from the ISP side - they are simply not being provided by your ISP's DHCP server. Could you please try to use custom DNS servers on your mobile interface, by configuring them via WebUI at Network > Interfaces > [Edit MOB1S1A1 interface] > Advanced Settings > Use custom DNS servers [enter 8.8.8.8 and/or 1.1.1.1 for testing purposes] > Save & Apply.
After doing this you should be able to have proper functionality of the passthrough/bridge mode as dnsmasq service should stop crashing. This issue is also going to be resolved from Teltonika end in the future firmware release, to make sure our dnsmasq service isn't impacted by incomplete configuration from ISP side.