Ok my problem was very similar I wasn't able to send any messages half the time and if I was connected to Wi-Fi I couldn't send them at all and I spent days and weeks coming the internet trying to find a solution and was not able to. So hypothetically I may have done some things I shouldn't have and in the process I discovered the solution to MY PROBLEM, I can't guarantee that this will fix your problem but it's something you can try. So go into Settings>About Phone and scroll down to where you see your Build Number and tap that box repeatedly (for my phone it was 15 taps) until it says that Developer Options enabled then go into the Developer Options and scroll down until you see Cellular Data Always Active and turn that option on then restart your phone and for me my problem was fixed! I hope that this method will help some of y'all, if it works post back and let us know for future answer seekers.