"Old tech"... heh, that's kinda like going to a restaurant and them saying "oh, we don't let you pick what you eat or how it's prepared... that's old fashioned. We just go ahead and give you food."

Yes, I can see why he might say that because in the old days you really "had" to do things that way, and now you can just plug in their "smart" modem/router/wifi AP combo, not configure anything, and magically have the interwebs in your house... but that's just for the masses who don't care.

This Primus page has some instructions on bridging a Primus DSL modem. No idea how it relates to the model your customer has.

[Link: primus.ca]

You'll have to scroll about 2/3 of the way down the page to the "SmartRG 505n Bridge Mode" heading.

No reason the Trendnet shouldn't be able to get "double NAT'd" internet if you turn off PPPoE on it, enable DHCP for all settings, and just configure it to work like you would with a standard cable modem. But that's slower, and useless for any kind of UPnP or port forwarding that may be needed.

Primus may not "officially" support it, ie. they don't want to have to provide connectivity support on your customer's router, but there should definitely be a way to bridge whatever modem is being used. Get the brand/model number to start.