After reading much complicated information about how to use the hAPac3 as simple Accesspoint connected via ethernet to a main router i discovered that their is a verry simple way that i like to share.

You just have to connect to your hAPac3 by an ethernetcabel that you plug in a ethernetport and the connectioncabel to the router in eht1.

Now you can use winbox for accessing the hAP via its MAC adress (neighbours).

In Winbox you can choose "Quick settings" and select the "WISP AP" mode. Select "Mode" bridge, choose static IP-Adress, choose an IP adress for management reasons for the device, type the network mask and give as Gateway the IP adress of your router. If it is another Mikrotik device its usualy 192.168.88.1, other brands use often 192.168.68.1.

choose now an network name and choose the country. If you want to have a password for the network enable WPA and WPA2 and typ the desired password in the box.

Typ "apply" - and thats it! You should now have an 5 GHZ network thats work.

Unfortnatly you have now just one 5 GHZ network. If you want to have a seperat 2 GHZ network you have to configure it manualy. This is done in the Wireless menue editing the deactivated "wlan1" device. Make shure you choose mode "ap-bridge", the right country and select "forwarding" in the wireless menue. Then you go to the bridge menue, select port and adding it even their.

If you want to use add a guest network with a diffrent password you have to go to the security tap of the wireless device dialog. By clicking "+" you can made a new profile and name it for exampel "guest". Choose "WPA PSK" and "WPA2 PSK" and type the same password in both dialog boxes called "pre-shared-key". Go back to the wifi interface list, hit + and choose type "virtual". Go to wireless settings, choose mode "ap-brigde", give it a name in the SSID box and choose a master interface. If you want a 5 GHZ guestnet chose your 5 GHZ name. Under security profile chose the one you have generated ("guest") and hit apply. Since this is a virtual net you do not have to border with the bridge.

I do not know if this has any disadvantages, but it was more failsafe that trying to follow the other posts in the forum.