Had the same problem. Changed the network selection to 3g and then selected SFRREUNION even though it said forbidden network and it connected. Then I changed the network selection back to 5g/4g/3g auto and the network selection to select automatically. Then it just worked.
You are not the first to highlight that sometimes, roaming onto a "Forbidden" network is the only way to get a connection, @dsm83 - https://community.o2.co.uk/t5/Pay-Monthly/Data-Roaming-California/m-p/1666184/highlight/true#M266308
Thanks for the tag @pgn
I'm currently getting this "Forbidden" network option checked out.
If I'm able to get some further information on this, I'll provide an update 👍
@pgn After discussion, I've had it confirmed that while roaming "forbidden networks" can still be manually selected if customers are struggling to access service on other available networks at their current location.
Forbidden networks are more for when you're at home, to prevent your SIM from connecting to another service provider 👍
Well, after 20 years of roaming I've learnt something new. Not that I've ever been in a situation where I've needed to manually select a network.
The face that @Dave-O2 has had to go away to find our means that most people won't know
We shall have to get people to check this out as I'm never too sure on answers from O2