Why my line work only with 3G/H+ and not with LTE?

Hi! I'm sorry for the English but my French is terrible.

I stay in Tunis and I've a problem with my mobile line.
When the mobile is under LTE network it's very slow! I touch max 1 Mb/s in Download and 10/20 Mb/s in Upload.

This problem appear ever, with max signal also. I've a Motorola G5S Plus but after a test with an iPhone 5S I can tell that the problem isn't my mobile because with the iPhone I had the same problem.

The configuration is right because 2months ago it worked correctly. Now I must disable, from mobile settings, the LTE for reach a sufficient speed.

In fact if I'm under H+ signal I can surf at 7/9 Mb/s and 2 Mb/s.

I wait a your reply because in this situation, if I don't solve fastly, I'm forced to change operator.

Thanks for your time.
Giuseppe S.