Sleep Number bed with no wifi and no app — does the firmness still adjust?

I will be the dissenting voice: I think the requirement you have described argues against buying this category at all. You want a firmness you can change and no dependency on a company existing in ten years, and that is a latex or foam mattress with a firmness you chose plus a topper, for less money and with nothing to pair. I went from an adjustable air bed to 20cm of latex over a slatted base and the only thing I miss is being able to change my mind at 2am, which I did roughly four times in three years. The pump is a single point of failure attached to a company's support policy.

Mine has run offline for four years and the firmness adjustment works entirely from the remote, because that is just the pump and the valves. What you lose is everything in the app: the sleep tracking, the reports, any automatic adjustment features, and firmware updates. The catch is that this varies by generation, and the newer models I have seen in showrooms lean much harder on the app for initial setup than mine did. Before you hand over money, get the model number off the pump and phone the manufacturer to ask specifically whether initial pairing requires the app, because that is the failure mode that bricks a secondhand one.

On the money side, be careful with used ones. The mattress part is essentially foam rails plus air chambers and it holds up, but pumps and remotes are the parts that fail and they are proprietary, so a replacement pump is a manufacturer part at manufacturer prices rather than a generic. When I priced mine a couple of years ago it was a meaningful fraction of what the whole used bed costs. Factor a pump replacement into your third of retail and see whether it still looks like a deal.

I bought one used with no remote because the seller said the app controls it. It would not pair without an account tied to the original owner's registration, support was polite and unhelpful, and I sold the whole thing on for less than I paid. Do not buy one without the remote in the box and a demonstration of it inflating both sides in front of you.

Get the model number, ring the maker, ask two questions: does setup require the app, and can I buy a pump in five years. If either answer is soft, walk.