Well EU roaming only works on Guido.
The only thing that I can suggest is that you contact 48 support and ask them if they can terminate your Don membership early (you'll lose any remaining membership allowances) so that you can buy a Guido membership and have that start immediately so that roaming works.
Hope this helps.
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I am not a 48 employee - if you need to contact 48 support you can do so by
chat or
email ([email protected]).