As per my previous post - that text message refers to a billing add-on which reduces the cost of roaming. Your phone still needs the roaming facility enabling, which allows access to foreign networks.
This is normally done by texting ROAMING to 150 before leaving the UK - did you do this? In the absence of confirmation otherwise, the assumption would be that you haven't. This would certainly explain your symptoms, and the only recourse now is contact CS from another means, be that a working phone or a web-calling app such as Skype over WiFi (use +44 800 0798586) and request roaming is enabled.
Brexit hasn't changed anything so far as enabling phones to work abroad is concerned, this has always been roaming. The only change as a result of Brexit is the cost of roaming, which is not an issue in your case as you have already chosen an add-on. What you don't appear to have done, is enable roaming - you would have needed to do this before Brexit just as much as you need to now.
