@Berndtan 

I would try the following in this situation, assuming heat pump supports WiFi 2.4GHz band:

1. Enable Guest Network;

2. Configure it to broadcast on 2.4GHz only, by disabling 5GHz band in Guest Network settings;

3. Leave password empty (you can do that for Guest Network);

4. Make very simple SSID for Guest Network, that consists of digits only, example: 123456

5. Set Guest Network WiFi security to None.

Try connecting heat pump to that simplest WiFi setup.

If that does not help, power off all Satellite Deco, leaving just Main Deco. You have removed Deco mesh and now run standalone router (Main Deco). Try connecting heat pump again.

Make sure you disabled Guest Network after that test ASAP, as it is very insecure connection opening entry to your home network for everyone. The point of that setup is to find if there are any WiFi settings impacting connectivity, by removing them all. If it helps, then you can start walking back one by one to find which of settings is a problem.

If that does not work, get the most inexpensive WiFi router you could find, in my region that would be TP-Link N300 WiFi Router (TL-WR841N) for about $20 US. I will use TP-Link N300 as an example. If you have spare router in house, just use what you have.

Connect that router by its WAN port to Ethernet port of Satellite Deco nearest to heat pump. Configure TP-Link N300 as router, with its SSID/password which should be different from what you have for Deco mesh.

If heat pump can't connect to SSID/password of TP-Link N300, you can now talk to panasonic dealer asking them why heat pump can't connect to generic WiFi router. You have removed Deco from the picture completely in that setup. If dealer is withing your reach, deliver them TP-Link N300 and ask them to test it with their heat pump equipment and tell you how to configure it properly to work with heat pump.

  @Alexandre. Hello, looked for my old router, connected it as you wrote, but could not find a new network in my Wi-Fi settings. Thought of giving up and restoring the whole thing. But I made one last attempt to connect the pump with the panasonic comfort cloud app. And wouldn't you believe it, the light on the pump stopped flashing and stayed on, and that I could continue with the settings, the app found the pump and I could register it, etc., let's hope it continues to work. I don't understand what happened, but the main thing is that it works. A very warm thank you to you for the support. Greetings from Gothenburg