Poor Signal

Not sure where to begin but been with T-Mobile for couple years using internet service. I have the silver trash can router. When I first got the modem it worked pretty good, I'm about 4 miles from tower, I live in a rural area with a lot of summer vacation homes, problem is people bring their T-Mobile routers from another area and use the tower near me for service. T-Mobile should lock modems to the area they are assign too. But that will probably not happen, so I want to get a new modem with the antenna ports and get a external antenna for better signal. Is this possible or would I have to buy a new one? I pay my bill early every month and don't use a lot of data. At 70 not much for me to do except stream a little tv. Verizon is coming to my area soon or just try and work this out with T-Mobile. In the two years never had a problem with billing or other support, its hard to find service like that these days. Any suggestions on getting a modem with external antenna ports apricated.