Cannot connect to shelly 1pm with browser

I have 8 shelly's 1 pm, but I cannot access them in the browser. The IP address is a fixed value given from the router (192.168.1. 32 to 39). When requesting the page it takes a long time until the browser gives an error message saying that the page cannot be loaded. In the router I can see that the shelly's are logged into the network and that the assigned IP addresses are also the ones I enter in the browser.

Tried several times to reset the shelly's (factory reset with button on shelly), but this only works for a short time. After about an hour the shelly's are no longer accessible through the browser. The latest firmware is installed. There is only 1 shelly 1pm that can be accessed from the browser. The router is no more than 1 meter away from the shelly's.

I also tried the android app, but even this way is was not possible to get in contact with the shelly's.

Using the app to set up the shelly's from the factory settings gives an error message that the device cannot log on to the network.

What can I do to access the shelly's again?

The used router is Asus AC1900U.

The router has 'Set AP isolated' option. I tried it with this option on and off. There is no difference, even after a reboot. A funny thing is that there is no problem to connect to the shelly dimmer.

Something crazy has happened. Instead of rebooting from the web interface I took the power from the router. Since then all shelly modules are accessible again.

So the problem is solved.