You will be in a tough spot if you're in the USA. All the nice Nokias and decent low-tech phones are rapidly becoming useless as carriers phase out 2g (and even 3g) coverage to free up bands for LTE and now 5g. If your "dumbphone" doesn't support VoLTE you'll be frustrated with dropped calls and spotty service everywhere. I know this from experience. I settled on T-Mobile's only "dumbphone" - the Alcatel GoFlip [1].…
Anecdote: I used to have an LG dumbphone with AT&T (I'd purchased it with prepaid time at the airport). A few years ago they sent me a message to come to their store and have it replaced for free because it would stop working with their planned "network updates".
I did that and got an Alcatel dumb phone. Not sure if it's using LTE or whatever G, but it works well and I'm reasonably happy with it. I do wish it was a flip-phone like the LG, because I've butt-dialed 911 a few times with it.
(They did initially give me what must've been a used phone, since it had over 50 personal contacts from someone else on it, but that's a different story...)