I have 2 * shelly Pro 1 controllers and they where just fine inserted into my new router.I have exactly the same problem. I have 2 Shelly 1 and an electrodragon that do not connect to my 2.4Ghz network. I've tried everything. With my other Synolgy RT-2600AC router they connect without problem with the same SSID and password with the same encryption. I have reset it from the factory several times without getting them to connect.
If someone finds a solution, please comment.
I also had the problem that the ONT signal was lost when turning off and on the router and I had to remove and put the cable so that it could reconnect to the internet. It was fixed in version 3.0.0.6.102_37094.
Yes I did the logging onto the controllers WLAN network and defining it that way to be part of the new setup - as said I had no problems here.