@Alexandre. I did the final test yesterday and disabled the Decos and only used my old WiFi router. Unfortunately everything worked fine and so I came to the conclusion that my setup is not...

@Alexandre. Again thank you for the clarification. In the meantime I have reconfigured my network. Now it looks like as follows: ZTE MC889 router (ISP) <--- (cable) ---> TP-Link WR841N <--- (cable)...

@Alexandre. Thank you again for your detailed answer. I will give option (3) a try. Before I do so, there is just one thing that confuses me. For option (3) you write: (a) Keep running ZTE MC889 5G...

@Alexandre. PS: I have a TL-WR841N router in my toolbox, which I have used earlier. Since it has only 2.4 GHz it is no longer used at the moment. But without using it's WLAN functionality it might...

@Alexandre. Thank you for your answer and your suggestions. I tried to find a manual of the ZTE MC889 online, which turned out to be unexpected difficult. There are tons of manuals describing how to...

@Alexandre. Before that, I only had a TP-Link TL-MR6400 router with a LTE simcard. It had the same port forwarding settings. The Raspi was connected to the old router exactly the same way (LAN cable)...