Hi,
What is the address of the main router connected to the switch and what's the subnet?
Have you changed RUT955's LAN IP accordingly so it would not overlap with the one that is connected to switch? You can also try disabling DHCP server on it.
Also, if you're connecting from switch port to WAN port of RUT955 - you will not be able to access the WebUI of RUT955, so maybe that's the reason? Though the internet connection should be there on RUT955 and IP should be leased from the main router.
EB.