Like Tesserax I am astounded you got 11 or so years out of Time Capsule.. Most of the first batch were dead in 18-24months with dead power supplies due to poor cooling. The old Airport Extreme not so much as it was a far superior design with external power supply. Just the same it is testament to how well it was built. Even commercial stuff is not really good for 10 years.
The time has come though to dump both. Wireless has moved on several generations in a decade.
we may need a new router since Apple is out of the business of airport time machine-Any advice?
Do you have the current setup linked by ethernet upstairs to downstairs or were you using wireless?
Much depends on the answer to that.
If you use ethernet it is much easier to setup a roaming network and get fast speeds than using wireless links.
If you are stuck with wireless then I would recommend you look at mesh like Netgear Orbi RBK50 which is usually the chart leader in the category. They are not cheap but will work far better than most. It also means you have ethernet ports available on both ends. Plus you can connect them by ethernet rather than wireless if you wish.
If you do have ethernet then Synology is worth a look. RT2600AC as router downstairs connected to the main modem or router.
High end Asus is another choice. Both brands can do mesh to similar products in their lineup. You always have to stick to single brand though. You can plug a large USB 3 drive into the Synology and use it for Time Machine if that is what you were doing with the Time Capsule. The Asus is also possible but it is not as reliable for TM although I think it is overall a better router.
Tell us what your requirements are and some idea of budget and we can suggest alternatives.. but is the link upstairs to downstairs ethernet??
What is the current modem? Make and model please.
Who is your ISP?