AT&T Grandfathered plan vs ISP?

[a-o-22120]

I've been using a grandfathered plan I got from someone on the site. Great service, but not perfect. The speed tends to dip around the afternoon time where I live. So I was considering to get a decent ISP plan for a more reliable and possibly stronger connection at all times. But then I started thinking more about the AT&T plan...there are routers/modems out there that can run on SIM cards right? So if I bought a powerful router/modem with a SIM slot, would that fix the dip I tend to get?

Any input is appreciated.

[bkb1942]

I am sure it will not help. My SIL works for AT&T and has to get his service from another provider because he needs faster service as he works from home a lot. They just don’t have the better service in a lot of areas.