Thank you for the information!
Perhaps you have at least one gateway for testing where the firmware could be updated and settings changed without losing access?
As for the troubleshoot file, if it will be attached to the original post, it will only be visible to Teltonika moderators. An alternative would be to send it via a private message. However, if that is not possible, it would be best if the system.log file could be attached, as that's where the entire logs are stored.
Also, can you verify that ping/wget and failover rules are disabled? If you're using a private APN these rules will not work, as the public internet will not be accessible.
If TRB140 is available for testing locally, then I'd recommend using RutOS v07.02.7 and doing a factory reset if it was not done, as I can see that there were some improvements to Multi-IMSI SIM handling since the version you're using.