purchasing used 5g gateway for home internet

I think it wouldn't make any difference where you get the gateway. If you get it for free from T-Mobile, it will come with a SIM that identifies it on the T-Mobile network. At the end of the summer, you'd cancel the service and drop the gateway off at a T-Mobile store.

If you buy your own on eBay, you'll still have to cancel the service and possibly return or destroy the SIM.

Unless I'm mistaken, you'd still have to pay a disconnect fee to a traditional ISP, even if you only use it for a couple of months out of the year.

I'd call Sales at T-Mobile and tell them you want to subscribe to the T-Mobile Home Internet service but you'll need to suspender it after you leave your summer home. See what they suggest. I've never had to deal with a scenario like yours, but I'll bet you're not the only T-Mobile customer that has this environment.

Another thought I just had: why not subscribe to T-Mobile Home Internet at your summer home, then pack it up and take it back to your residence for use there? That way you'd never have to return it. The downside of this idea is that any Internet device in your regular residence will be cut off from being online as long as you're away, and that may pose a problem for you.

Hope this helps! Good luck!