The Ethernet port on the AirPort Express has a top speed of 100 Mbps, so that in theory is the speed of which an Internet connection is capable using an AirPort Express. Realistically, because there is always some loss, the AirPort Express might provide 75-80 Mbps speeds.
So, while your Internet speed plan might have been 200 Mbps, the Express was delivering less than half of the speed of your "old" plan. It does not really matter that the speed been doubled to 400 Mbps, the performance of your Express remains the same as before.....about 75-80 Mbps.
As far as a new router, virtually any router will be compatible with Apple products, because all routers must meet industry standards. If you are the type of person that likes to keep things simple, our recommendation would be to first check with Spectrum to see what they can provide to you. The advantage here will be little to no setup required on your part and Spectrum will offer support for the router in case of any issues.
If you opt to purchase another router, Spectrum will provide no support for the product if there are any issues, and instead blame the router for the issues.
If you are not the type of person who likes to keep things simple, then any of the Top 10 Routers for use with Spectrum should meet your needs. Note that most of these products are both a modem and a router, so you would not need to use a separate modem and separate modem in most cases.
https://www.comparaboo.com/routers-compatible-with-spectrum?origin=google&google_params[matchtype]=b&google_params[network]=g&google_params[device]=c&google_params[creative]=482346168231&google_params[keyword]=&google_params[adposition]=&google_params[adgroupid]=106062690931&google_params[campaignid]=9881114630&bs=&google_params[feeditemid]=&google_params[targetid]=dsa-19959388920&google_params[loc_interest_ms]=&google_params[loc_physical_ms]=9027202&google_params[devicemodel]=&google_params[target]=&new_api=true&dest=0&sys_id=0%7C894&gclid=EAIaIQobChMImpaZwpvd7gIViuHACh2CIwTQEAAYAiAAEgJEAfD_BwE
