I didn't notice the router model was already given. Sorry for any confusion I may have caused.

Yep I’m in Japan.

Probably. But KPN uses it’s own software - of which the last version does not seem to play ball with Sonos….

Googling the router model suggests it’s from Japan.

Since I do not see a country mentioned in your public account details: are you Dutch? Using a KPN V10a router?

Thanx for your help.

I will try to troubleshoot again on my day off.

Then it’s most probably the router that’s not cooperating. 

I have a Sonos app on my phone, my tablet and my computer. Whatever device I choose as long as it is connected with 5ghz the app can't find Sonos.

It sounds like there’s either something odd about your router’s 5GHz band, or something funny with the mobile.

Did that already and still the same unfortunately.

As an experiment wire one of the Play:1s to the router.

My router is Aterm WG1200HS4,

and Multicast was already enabled.

What router is it? It may have an option to enable multicasts/broadcasts between network segments. 

Yes Sonos connects via WiFi.
I have two Play1’s.

Does Sonos connect via the WiFi, or is there a wired Sonos component?

Some routers don’t correctly forward discovery broadcasts between the two bands, preventing a Sonos controller on 5GHz seeing players on the 2.4GHz WiFi. Often wiring a single Sonos device resolves the problem as the system then uses its own dedicated wireless, connecting back through the wired unit.

My wifi setup is simple. only one router. no mesh.

The only change I made was the router. 
A couple of months back I replaced the old one with a IPV6 compatible router.

But I am not sure if the issue started either before or after.

Have you changed your WiFi setup?

Describe your network for us. Are you using any 3rd party wireless mesh units? What is wired? What is wireless?