Any combo modem router is typically trash and you NEED separate modem because if you get a combo you will be in the same situation. They will flash the combo unit with the same firmware wether you own it or not.
You will want an aris modem from there approved list and a good wireless router. When you swap out your modem you will need to call in so they can flash it with thier firmware (which is fine). You can then configure your router as needed.
I can't recommend a wireless router because I have a Unifi household and have been out of the consumer space for a while. I hear netgear nighthawk are still creame of the crop though.