Do you mean you've enabled traffic using this method?: [Only registered and activated users can see links. ]. If so, i've also found that GTM20/21 can be a problem to successfully add new subscriptions for use with nuvi 6xx and 7xx etc. You may have better result if you use the other method to extend the expiry date for an original 'short-term' [not Lifetime] TTN USA subscription which has already expired [scroll down to "USE THIS TOOL ONLY TO EXTEND EXPIRING SUBSCRIPTIONS" at the base of the initial post of the above thread.
Unfortunately to add another problem i think that subscriptions on the older [pre-GTM25] TMC receivers are also somehow tied to a specific device as well as the traffic receiver.