cant connect to interneet while roaming

Issues with roaming is something you need to contact your cellular provider about, and I suggest you do so to try to remedy this. Each provider may have different services that they support when your phone is roaming.

Again, cellular data is provided by your cellular provider. You need to work out this issue with them.

However, one thing you can try on your iPhone is to reset its network settings to see if this can help.

To do so: tap on Settings > General > Transfer or Reset iPhone > Reset > Reset Network Settings