The Time Capsule was....and still is.....the only product that combined a router and a built in hard drive, so If you were happy with the Time Capsule, you might consider buying another. Products are still available as new, refurbished and used on Ebay. Some of the other online stores may still have product as well.
Otherwise, you will need to look for a WiFi router that will support Time Machine at the USB port on the device and add a separate USB drive for backups. Synology and the upper end Asus routers qualify, and there may be others.
Another option would be to purchase a new WiFi router and connect a Network Attached Storage (NAS) device to one of the Ethernet ports on the router. You will be able to back up over the WiFi to the NAS. Synology is the NAS brand of choice among most of the regular contributors to the support forums, but there may be other devices and brands that will work as well. Just make sure that the NAS will support Time Machine, as some do not.
Stay tuned for more answers from other users as well.