Dual Sim: Only one Sim card working at a time

Hello,
I have a strange problem with my HTC Desire 12+ which I am using as a dual sim for several years now without any problems. I recently changed one of my service providers and consequentially got a new SIM card which I inserted in one of the two slots.
In contrast to my old SIM card, where the dual sim was working fine, I now can only use one SIM card at a time. More specifically, I am not connected to the mobile network of one of the two SIM cards even though both SIM cards are activated (see screenshot below). Consequentially, I cannot receive or make calls with the SIM card not connected. When I deactivate the SIM card which receives a signal, the other one connects to the network. But it automatically disconnects once I turn the first one on again.
I already tried to switch them in the slots, to disable automatic search for service provider and to force 2G for one of the SIM cards. None of this helped. Does somebody has an idea?
Thank you very much in advance!

Details of my device:
HTC Desire 12+
Not rooted
Android 8.0.0
Service providers: La Poste Mobile (France) & Aldi Talk (Germany)