I was able to solve this problem by setting up two profiles and adding a second wifi network.
The first profile is used whenever I want to connect the RUT955 to another wifi network: This profile is deactivating the SIM (actiually switching from SIM 1 to SIM 2, while there is no card in slot 2), and activating station mode.
The second profile activates SIM card 1, and deactivates station mode.
I encountered the same problem with the accessibility of the wifi network after restarting the router or changing profiles. I therefore added a second SSID. Now, I am able to connect to the router's WebUI, even when one is not available.
Btw, this hint works for me only with firmware versions 6.08.3 and 6.08.5, as my RUT955 does not work with 7.00 and a Telekom Deutschland SIM card.