I've guided dozens of people through the activation procedure and I have never seen similar issues so far. There are also two reports from people who couldn't get their SIMs activated at
Registered but no service - Prepaid Wireless Internet Access.
I assume they had some system maintenance or so - maybe this trial month, of which I hear for the first time, is a result of this.
Your report about the trial month, about which I hear for the first time, gave me a hint of why your tariff change by SMS failed: Lidl seem to have introduced their own number for SMS subscriptions which is 54356 (LIDLM) while before they shared the number with Fonic (36642 = FONIC).