If you are talking about AP, you probably thinking about connecting both RUT955 via WiFi. In that case, Ethernet cable is not required. Just simply navigate Network > Wireless and enable AP mode in one router. In the second router, please navigate Network > WAN, enable WiFi as primary/secondary WAN source, and press the scan button. Once you will see your other router name, press Join Network and enter WiFi password.
If you want to connect your devices via Ethernet cable, please insert one cable end to the LAN port of the router that does have an internet connection and another end to the WAN port of the secondary router.