Going back to the OP post, Id be more inclined to guess they have a not too great IPTV sub, and that changing box might not make things any better.
My closed group IPTV provider has been about for 4 years, and Ive used them for 2, with the amount of issues encountered being on one hand.
And, over that time Ive used even 1st gen Firesticks with 1080p 60fps US feeds perfectly, so the buffering will almost certainly be down to the providers feed.
My set up just now is a Fire stick, 2nd gen (with Alexa voice), a 4k stick, and two 2nd gen Fire tv 4k boxes (the good ones with the usb, microSD and ethernet sockets) - all with zero buffering.