When Apple has introduced a new connectivity generation in the past, e.g. 2G to 3G and then 3G to 4G, it has on both occasions at least initially had a toggle in Settings to disable the latest technology.
As a matter of fact you can still do that, the setting is different but you can now chose between LTE and VoLTE, just LTE, 3G or even 2G.
Some carriers may even charge for 5G like they did with 4G so if you don't pay you won't be able to connect to the 5G antennas even if you have a compatible phone.
Actually I don't care about 5G now, I'm confident they'll put it on some iPhone models in 2020 (at least the Pro) but I can do without it for another couple of years, so if they offer a 4G only modem this year I may buy it if it is a good upgrade from my Xs