MTS has generic instructions on their site for how to configure third party routers to work with Ultimate. Simply search "MTS third party router" and the link should come up in the results.
I could be silly and do one of those "let me google that for you" but I won't
Basically you go into your router settings (before plugging it into MTS gateway otherwise you might not be able to access your router settings page) and turn off DHCP server for the LAN, change the router IP to 192.168.100.200, and plug the wire from the RG into a LAN port instead of the WAN port.
I've seen other people say to do it differently, and I can't see why. I'm no expert though.
The plug on the back of the VIP1200s are only meant for a feed from the gateway, not for Internet. Obviously it works but is not supported by MTS. Assuming it is because of the problems you encounter, possibly causing an unnecessary repair call.
Hope that helps!
Authorized Service Provider (Contractor) for MTS Allstream. Any views expressed are my own.