How should a personal router connecting to Xfinity Gateway be set up?

Just put the gateway into bridge mode so that it acts only as a straight cable modem and you don't have an undesirable "double NAT" condition on your home network (you can Google "double NAT" for info about its potential pitfalls). 
 
Here's how;

https://www.xfinity.com/support/articles/wireless-gateway-enable-disable-bridge-mode 

Good evening @user_xi55c7 and thank you for reaching out to our dedicated Communities team on our Forums, we appreciate it and hope you are otherwise having a wonderful week so far! Thank you for letting us know that you have one of our awesome XFINITY gateways set up but am sorry to hear that the signal isn't great in your room. How far is your room from your gateway? 

We understand how important it is to be able to extend your coverage throughout your home and assure you that our team is here to help address any connection issues you are experiencing. How are your speeds currently performing? Have you tried any troubleshooting steps yet using our user-friendly and convenient XFINITY app? You can try some basic troubleshooting by following our instructions listed here https://www.xfinity.com/support/articles/internet-connectivity-troubleshooting to see if that helps. 

May I also ask if you are an xFi Complete subscriber? If you're an existing customer who signs up for xFi Complete you'll be eligible for a 14-day assessment period while our Whole Home WiFi evaluation evaluates your network performance. If recommended, you could be eligible for one of our awesome WiFi Boost pods at no cost! For more information, I have included the following link https://www.xfinity.com/support/articles/whole-home-wifi-evaluation.

Our WiFi Boost Pod works with your gateway to extend your WiFi network, and help eliminate deadspots for more consistent coverage to keep you seamlessly connected throughout your home. They can also be purchased directly from our website here https://www.xfinity.com/learn/internet-service/wifi/xfi-pod.

Oh brother.................