Hello,
Could you explain why do you need TRB connected to your PC in the first place, if the allowed data usage is so low?
You could try to disable forwarding from LAN zone to WAN in Network -> Firewall -> General settings Zones section by removing WAN from inter-zone forwarding.
Or, considering the really small amount of data you have allowed to consume, stop mobile interface, if it is not used, either via Network -> Interfaces section in the WebUI or via CLI with command /etc/init.d/gsmd disable.
Best regards,