Why my APN doesn't works after changing country and carrier?

[Windows Central Question]

Hi!

Informations:
Lumia 930 (green)
RM-1045_1011
000-FR
15053.26000

So, I think you got most of infos to help me ^^

I've recently changed country, and I've bought a new SIM to put in on my L930

I've read somewhere it should help to do a hard reset: done, without any result.

I've tried to configure my APN and MMS APN on my new SIM, but it doesn't work and I can't manage to understand why...
I've enabled roaming, switched roaming to Manual and National...

Is this related to the Variant Country Code? Should I Flash the ROM to one from my new country?

My father have a Lumia 650 and have the same problem...

Help me, I don't want to leave WP, I don't want to join the green robot or the apple OS...

Thanks in advance for your help!