The service might not be available unti iOS 18 releases. The 15 has satellite connection as you found out since you tested it successfully. I am not sure if the texting will be made available for the 15, it might end up pro exclusive. Although I saw no such suggestion so likely you will be able to use it starting with iOS 18 in September when it comes out. If not, you will always be able to use it for an emergency - but please don't rely it on a beta iOS.
If you are ever outside cell reception and have an emergency and just assume the beta will work with the satellite you might end up in a bad situation if the beta broke it.
You need to have a SIM installed, active and airplane mode needs to be disabled. You might want to activate wifi too. Only in these cases can satellite be used. The reason is that Apple wants you to use this service only when you really need it and not just for fun even though you could use your SIM card.
If Apple told you messaging won't be available on the 15 then it might really be limited to the Pro. You can check it probably by watching the iOS 18 announcement video, they might have mentioned it.