What are the benefits to attaching a router to our T-Mobile gateway?

We are evaluating a T-Mobile gateway device, which according to my son, is a glorified hotspot . Our previous internet is a Cox provided modem/router. My Epson WorkForce Pro printer wants to talk with a router and doesn't recognize our T-Mobile network. My son suggests getting a router to attach to the T-Mobile gateway, so he can access our network from outside our household and hopefully it will enable the printer. (We can save over $80USD by switching from Cox to T-Mobile.) Questions: What are the benefits to attaching a router to our T-Mobile gateway? Will attaching a router to the T-Mobile

Question by Thomas Matthews on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.