@twoj
it is clear what you need:
xFinity Router in bridge mode, if it exists for this type and your ISP allows it
or you mention a modem (Arris modem) that does not contain NAT per se and you get a public IP directly
the difference between the measurements is very large approx. 900 and 400
we didn't get ahead professionally, because this difference is not justified by the dual -NAT throughput, so there is still a cat hiding somewhere in the bag
if you have the opportunity to exchange, please come back to us afterwards (the curiosity moves the whole world )