Can't get or send iMessage

Reviewed all past replies but none fit. I HAVE a 5G connection but no WiFi. Can't send or receive text messages unless I either connect to WiFi or turn OFF iMessage (a bit leery of connecting to public WiFi). I understand iMessage needs an internet link but why can't iMessage be smart enough to know I don't have WiFi (but DO have 5G) and switch from iMessage to standard SMS???