RUT955: sim_select GPIO

I have a fleet of RUT955s, running official openwrt. According to the general specs, it should be possible to switch from SIM1 to SIM2 or vice versa, using GPIO 503. Unfortunately, this works sometimes, and quite often the modem reports “No SIM inserted” after switching. What is the correct procedure here, as I toggle RESET_N GPIO 507 after selecting the new SIM. Otherwise, where to find some drawing about the SIM-connection and its switch in the RUT955 ?

Pls, provide electrical drawing, describing connection between EC25 and SIM1/2. Otherwise: Which method to “restart” the EC25, to initiate SIM connectivy ?
I see 3 variants:
Toggling PWRKEY
Toggling RESET_N