It seems like the internal modem in the RUT240 keeps disconnecting from the mobile network. Were these intentional disconnections?
If not, I'd like to ask you to disable the IPsec instance on the RUT240 and leave the router running, to check if it could be related to IPsec.
I am not sure if this is what is causing the modem to disconnect, ]however, the timing between the IPsec tunnel establishment and mobile connection. The mobile connection can also be monitored by navigating to Status → Network → Mobile.
If possible, I would also like to ask you to factory reset the device (System → Backup), reconfigure the IPsec instance, and check if the behavior reappears. Thank you.