I had a similar issue with my Samsung Galaxy S7 and thought a phone upgrade would fix it. I've since purchased a Pixel 3a and I'm still experiencing the same problem, must be a network related issue.
When connected to wi-fi and I try to send a text message, the message won't send but will say "Sending" for 5-10 minutes before I get a notification that the text was never sent. I attempt to retry sending but the same issue occurs.
My system is up to date, I'm using the standard messaging app. If I turn off my wi-fi while the message is "sending" it goes right through. I'm always connected to Wifi at home and at work so now I may go hours without realizing my text never sent. This seems to be happening with all my contacts.
We always want you to be able to send/receive text messages. Can you receive texts on Wi-Fi? Have you tried restarting your router and device? Please connect to another Wi-Fi network and try this again. EricaG_VZW