Great, but the Echos can connect to both 2.4G AND 5G (So, that really was not the problem -- rebooting likely fixed the problem.).
Nevertheless. It is best to connect the Echos to 2.4G, because it is a low-bandwidth requirement audio-only device, so connect only streaming VIDEO devices to 5G. I keep my half-dozen Echo devices only on 2.4G, but iPhones, iPads, and Fire TV sticks, etc) are all connected to 5G for the wider bandwidth they need to stream quality video. 30+ devices connected to home network and all work great.
Xfinity should have set up (and designated) separate 2.4G and 5G signals for you. (And, told you how to use them!) If they didn't; shame on the cable guy! I also have Xfinity cable, but NOT their rental router/modem. I own my own router and modem. My experiences with the cable guy installers has never been good. Most are poorly trained with little experience.