Hello,
To start, do both modems acquire data connectivity? To check data connection state, go to Status -> Network -> Mobile section in the WebUI. Keep in mind that load balancing must be enabled in order to have both SIMs operational at the same time.
If you have recently performed a firmware update, depending on the previous version, you might need to try resetting the router to factory default settings. To do so, navigate to System -> Backup -> Restore default settings.
Since the same package mwan3 is used for failover/load balancing management, disabling interfaces affects both instances, however, the device can only operate in load balancing or failover mode, it cannot do both at the same time. What you are disabling is interface, participating in load balancing or failover.
In terms of configuration it is really straight forward. Simply choose the mode depending on your use case, either load balancing or failover, enable available WAN interfaces, set their metric or traffic ratio.
Best regards,
