with the newest firmware RUT9XX_R_00.06.06.1 the input form for multiple access point setup (WAN via WiFi) does not support to join networks with dots for example NSA.Surveillance.VAN would be not possible but NSASurveillanceVAN
I found a workaround:
Setup the network without dots, ssh as root to your RUT955, edit /etc/config/multi_wifi: insert the dot(s) in your SSID. Save it, reboot or restart wifi and wait until it's connected. Then you can edit the SSID in Network/WLAN and save it. Not you have a red marked box in multiple AP setup but it works.