Hi,
As RUTX09 uses only one modem for both SIM cards, when the SIM switch happens - SIM1 is swapped into SIM2, SIM1 stays offline and won't do any connection checks as SIM2 is the primary one right now. As it is only one modem router - it will have no way to check if SIM1 has data connection or a network available and for the check to happen - SIM2 would need to go down for SIM1 to get up (and that does not guarantee that SIM1 works).
So going back to SIM1 is kind of impossible when SIM1 is not active and there is no way to check it. The only option is possible is to force SIM2 to switch to SIM1 once SIM2 goes down.
If you wish to use both SIM cards and have them both active at the same time you will need to have RUTX12.
EB.