It happens to me, among other things, that they are connected and do not appear connected to me. I also have different devices that don't connect no matter how hard I try. I put another router with the same configuration and these devices connect without problems.
Sorry, I don't know if you have passed this information, have you tried to update to the latest original version? There was a problem with the ONT in the WAN input that when it restarts the router loses the connection and if you remove and put the cable it is solved. With the update this no...
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...