Compare WiFi speeds of rented XB3 DPC3941T cable modem/router combo to the newer AC1900 models.

I am getting ready to start streaming and I need a fast reliable WiFi. I currently have the rented XB3 DPC3941T cable modem/router combo because I love the home phone option. I am not too tech-savvy but I realize that most people are telling me I need at least AC1900 specs. I also realize that this combo is several years old and it probably doesn't meet or exceed these specs. I have searched for specs that made sense or at least addressed similar specifications, but all I get is a handful of Greek figures and empty promises telling me it is capable of speeds in excess of 700mps (and we all know this doesn't happen).

Do you know how the AC1900 protocol speeds compare to this model?
If it is significantly faster and since I want to keep the home phone service (so I need a modem with this feature), can I simply use one of the ethernet wired plugs and connect it to an external WiFi router to bump my speeds?