backup file

> Was wondering if I could use my backup file from my C7800 router to
> quickly configure my new router AX2700.

   "AX2700" is a speed, not a model number.  Look for "Model" on the
product label.

   Generally, the configuration files for these devices are specific to
the model, so: No.

   You could try it, but I'd expect the attempt to fail.