Activating new modem (owned)

Hello. I have just purchased an Arris SBG8300 after seeing it on the Xfinity approved equipment list, because I want to upgrade my plan to 400Mb and was told I would need a new gateway for these speeds.

I was unable to activate it with the mobile app (unknown error), and called Support for help. The agent told me I must return the old modem (Comcast-leased) before she could activate my new one. 

Really?

In this time of a pandemic, when our children need the internet to attend their classes remotely, and we rely on the internet to work because our offices are closed? You're forcing me to disconnect and take this equipment to a local Xfinity store during business hours - which means no internet during my work hours and/or kid's school hours, before my new modem can be activated?!? And I should just hope that the activation goes well, and I am only without service for a couple of hours? 

I must ask that someone help me with this, and find a way to make an exception. I want to activate this new modem and assert that everything is working well before returning the leased equipment. To ask otherwise is to be out of touch with the needs of your customers during this unprecedented time, when connectivity to the internet is an absolute necessity.

Failing this, my alternative is to switch to another provider to keep the disruption of service as minimal as possible. 

Thank you for your assistance.