Small update on BIMGo. I still haven't been able to call them yet, but from what I understand from
Blau lanza BIMgo, su tarifa plana de Internet móvil it really is a type of prepaid monthly internet plan. If you want to keep using it after a months useage, you just need to make sure you have the 21,50 in your prepaid account on the first day of the following month for them to debit. If it is not there, it will not work. Simple as that.
I'm still no wiser how you set one of these up. I presume any normal Blau SIM can be 'converted' to this tariff by calling customer services. Since it is prepaid, you just need to keep it topped up.
Also, in reply to the question above about APN: See:
blau - Preguntas frecuentes