Can connect Shelly1 but cannot fin in app

Hi, I had a working setup of 4 Shelly1's that operated perfectly on my skylight.

Because of safety I wanted to add them to my (new) guest network. Therefor I reset, and wanted to add them in app, while connected to my guest network, by by clicking the 'Add device' button. After scanning the network, the module is found and included on the same network. Normally I then get a in app message that devices are discovered, but that does not happen.

Checking my router, I can see the module is successfully added to the network.

Then I tried adding it by using the AP mode, same result

After that I tried the 'Add device by IP' button, after entering the IP it comes up with 'existing_device' message.

On my laptop I get the same results.
After 3 afternoons of add/reset/add/reset/add/reset I really get frustrated, grrrrr!

Btw, I had a VPN running on my phone, but removing that app didn't help

My router is a Experiabox V12 (KPN) aka Sagemcom F5359, restarted is several time and gave it a complete factory reset.

Have the same issue with the Shelly plugs

Already added devices work flawlessly

EDIT: Suddenly the Shelly1's came online, without any input from my side  
Than I switched from my guest to the standard network and everything was gone again. Switching back to guest did not resolve the problem, everything (expect the already registered modules) stay's off line.

To use all the potential of the forum here, see the following

In Deuitsch antworten is auch okay

In het Nederlands antwoorden is ook goed