If it were me, I would downgrade to the Residential 200 Mbps service (or even 100 Mbps) and just send the Mini back. Buy one outright if you need it for travel. The $249 purchase price is a one-time fee, and gives you flexibility from having to use Standby Mode. You will be able to keep your service cancelled/inactive most of the time ($0/month). For months where you need to use it, you pay for a Roam plan. Then it goes back to cancelled.
Alternatively, keep the Mini rental. They will allow this if you keep paying the $10/month Standby Mode fee on the Mini.