N600 DGND3700v2 router/modem & Nighthawk Mesh AX1800 Router

My wife came home with the AX1800 MK63 3 pack today thinking it would replace our N600 DGND3700v2 router/modem. We are trying to improve our not-so-good internet speed. Our service is through Century Link and is not cable or fiber, which I expect means we need a modem. We connect the DGND3700v2 to the phone line. Sometimes our service is good and sometimes very slow. We are not sure if the router might be the problem and thought we would start there. There are times when 10+ devices can be working on the DGND3700v2 at the same time. There are times where we can watch Netflix on the TV and Youtube on 1 or 2 phones at the same time. When service is slow, we reset the router to see if that will help. Sometimes it does. Sometimes it doesn't reconnect to the internet. It does connect to the devices. It can take a few times resetting it before it connects to the internet unless we wait for 10+ minutes. When it does, it takes 2-5+ minutes to finally connect and we all sit and wait. My wife thought the Nighthawk AX1800 would directly replace the DGND3700v2, but the AX1800 does not have a modem. I figure that my options are to stay with the DGND3700v2, connect the AX1800 to the DGND3700v2, or buy another separate modem.Would connecting to the DGND3700v2 improve anything since it's still in the chain or is the modem part faster than th router part? Is the DGND3700v2 outdated and should it be replaced? I can't remember how old it is, but I'm sure it's several years old or more.