My iphone has been in “no service” for days

I’m using an IPhone 5S and have been using lycamobile sim card for months, but it showed ‘no service’ since 5 days ago. I also have tried a different lycamobile sim card and it still shows no service, but when I used a sim card from another carrier it worked just fine. I have went through the lycamobile website and it says that if the connection works for others but not for them, the problem is with the phone, not them. So what do I need to do?