After changing mobile subscription roaming in Germany is bad

Dear KPN Support,

I am currently in Germany (Kulmbach, Bavaria region) using my KPN SIM card. Before changing the subscription to a more expensive one (this was in the twenties of April), my phone was able to connect to the Telekom.de network, and the mobile internet speed was very good.

However, after changing my subscription, I noticed, my device can only connect to Vodafone.de. The connection is very slow, and I am unable to manually select Telekom.de or O2.de — the system either shows them as unavailable or rejects registration.

Could you please check if there has been any change in the roaming agreements with Telekom.de in my region, or if my SIM profile has been restricted to Vodafone only?

I kindly ask you to restore access to Telekom.de if possible or provide a workaround.

Thank you in advance for your assistance.