Thank you for reaching out!
As I understand, you correctly configured SIM Switch rules for SIM1, and as soon as it loses connection, the router switches to SIM2.
To achieve the switch back to SIM1, the SIM Switch rule would need to be configured for SIM2 (or non-default SIM): Switch back to the default SIM card after the timeout.
This rule will check, whether the rule that caused the primary switch is no longer satisfied, and if so, make the switch back to the primary SIM card after the specified timeout.
More information about SIM Switch for RUTX11 can be found here.
Let me know if there is anything else I can help with!