Questions on how to proceed with initial set-up of the orbi RBR852 in a unique situation??

So my RBK852 was delivered the day before yesterday but I had already put in an order with my ISP Charter Communications Spectrum Internet service to upgrade my service from their 200 Mbps to the Spectrum Gig Plan...That hardware was delivered today. 
However, despite me telling the sales rep on the phone I would have my own router he said I had to use theirs for the initial set-up.,.Is this true?

If so, do I just follow the "Self Installation" Instructions & once I installed everything & had Spectrum activate the faster service then I just follow the set up guide for the RBK852 as if the service had been there & functioning for months or years? 

...I'm no expert when it comes to this stuff & Im also physically disabled living alone so I try to have a game plan before I do most anything...I know this equipment can get screwed up & start a nightmare of issues.

ive unboxed but not started setting anything up. Thanks for any advice.

The new Spectrum modem is model ET2251

And the Spectrum router is model SAX1V1K

Please let me know if you need something else.