Which brand of ATA do you have?
From what I can tell is that the ATA connects to the router with an Ethernet cable and that a landline phone connects to the ATA. How do I connect the other landline phones in the house?
My Verizon landline service jumped by 25% this last month.
I'm not the user whom your question is addressed to, but I can share my experience / setup. I am using Obihai Obi100, which I've bought for about $50 several years ago. There is the Obi200 series in the meantime, if you can get a good pricing on that, go for it.
In general, you can use any ATA brands and models. Obihai is a bit geeky / the UI feels like for more tech-oriented people, and very granular (many configuration details). Other good brands are: Cisco (they had a Sipura brand in the past, I think the Cisco VoIP team left Cisco and spun off Obihai), Grandstream etc.
Try to avoid built-in phone + ATA combinations (like those digital phones you see in the movies when they show business offices), because they are harder to upgrade or configure, may require a central server or PoE (Power Over Ethernet). Discrete components (like in PCs and routers) are always preferred, for flexibility and easy replacement of only once component should it fail.
A separate, little ATA box is very portable and you can carry it with you even when you travel.
For multi-line phone setups, the easiest route is to by a DECT 6.0 phone set, with 1 x base station + n x handsets. Or, if you prefer a wired RJ11 setup, buy an ATA with more analog (RJ11 connectors) ports.