cannot send sms

Hi everyone, this has been a week now that I can't send SMS properly. I can send imessage but when I have to text my friends who do not use iphone, then the problem arises. When I text them, on my phone it would say 'not delivered'. After that either my contact received the text dozens of times (same one although I only sent one) or they didn't get it at all and only get the message several hours/days later. On my side, some texts were not sent to me on time either. On Monday for example, I received a text sent on Friday!

I have contacted my service provider, and since then I have : 1. restore my iphone, 2. reset the network, 3. reset the phone! 4. turn on and off imessage 5. Check that my number appear properly and all other things I read on the internet! but NADA, nothing happened and I still have this problem! What should I do?😟

Thank you