Reception network bad, sometimes no reception at all, next hour 4 dots

Since 2 days, my iPhone is having this intermittent failure. First, i thought that maybe my provider is having problems, but when I put my sim into an oldskool (no smart)phone, I have great reception all the time.

When I try it in my iPhone, it usually gives "no service" or "my provider" with 1 dot of reception or no dots at all... Impossible to make or get calls at that time. Strange thing is, next hour I look at the iPhone which I didn't touch, and reception is 4 dots and calling is no problem. Then the dots of reception are decreasing and same problem.. (it can work fine for hours, and be in failure for hours as well..)

Can anybody help me with this issue?

Things I have tried so far:

-Restoring software

-Resetting network settings

-Hard reboot

-Playing with airplane mode

Thank you in advance!