Deutsche Telekom claims to block the update only for phones they sold: They use the IMEI of their sold Fairphones to provide an own version of the update.
I bought my phone from Fairphone directly, now using a SIM from a provider that uses the Telekom network. Yet, I still dont get the update. According to Deutsche Telekom support, that must be done by Fairphone themselves then, without involvement of Deutsche Telekom.
All in German, but at least public: Freigabe des Sicherheitspatches für Faiphone 5 | Telekom hilft Community
Now the next step would be to contact Fairphone support I guess. Currently not motivated due to months waiting time