The Keepgo SIM is able to connect to the Telekom (T-Mobile) network and shows signal (EDGE). However, there is no data connection at all, so the network is effectively not usable.
At the same time, Vodafone and O2 work without any issues (4G and data connection available).
Since Keepgo advertises Telekom as a supported roaming partner in Germany, this behavior is quite misleading in practice. While network registration is possible, actual data usage on Telekom is not.
For users, this effectively means that Telekom cannot be used with this SIM, despite being listed as a supported network.