Thanks for coming back to me via private message with your information.
I've taken a look at your account and all is fine from our network perspective. The roaming is fully active and there is no issues that we can see.
In terms of the forbidden message you are seeing, these will be the networks where we don't have a roaming agreement with. I'm wondering if the ones we do have an agreement with, the mobile phone itself cannot see them.
I can see that someone spoke to our teams yesterday and they did try turning the roaming off and back on so it's worth giving the phone another reboot just to see if this helps things.
If not, try once again to do a manual selectin. You may need to try a few networks in order to find one that works.
Finally, as a last resort, if possible, try your SIM in another handset to see if it connects.
Keep us posted with how you get on.