Shelly plug S not visible in app

I've replaced my router from a version with only 2.4ghz (KPN experiabox V8) to a version with 2.4 and 5ghz WiFi (KPN experiabox V10a). With the V8 I did not have any issues with the shelly plugs, but with the new router not one of them is trying to connect to the network. Is the Shelly plug S not able to pick up the 2.4ghz signal automatically?

For your information, I have connected multiple devices successfully to both 2.4ghz and 5ghz on the new router, but I can't disable 5ghz to test it on 2.4ghz only on the new router.

Thanks for your suggestion, but unfortunately it didn't help. I've changed de SSID and password on the new router to the original values of the old router, but I still get the message 'Could not find new devices to include! Please, first plug in your Shelly device!'