@David-TP Hi David, Vodafone recently replaced the Cable-Modem since they suspected it to be the culprit regarding connection issues. It turned out to be another problem but they changed it anyway. At this occaison they seem to have switched off Bride Mode without telling me. So you were right, there was a NAT Firewall involved after all. I checked online to make a screenshot of the Bridge Mode being activated and it was disabled. Sneaky mothers a Vodafone. I switched it back on. The Deco shows the public IP under IPv4 now. Portforwarding works like a charm now. Thanks again for your help. 

  @hquant Hi! How did you added the synology to your deco mesh? I connected mine with an ethernet cable to the main deco unit and can't find it. 

  @Flo33 I have TP- link 8port gigabit switch connected to the main Deco and the Synology Play 218 connected to the switch, Works fine, Tehe Synology is visible by IP, quickconnect and domain name.

Any chance you had a fixed IP set up on your synology before you switched to the Deco? I had that problem at first. Connecting the Synology to the old router an deactivating fixed IP fixed it.

  @hquant I just have the isp router and the deco is connected to the isp router, i was hopping to put an ethernet cable from the nas directly in the deco main unit ( seeing that it has 3 ports, 1 i use for the wan) and i switched my tv also on the deco network so i can have high transfer speeds from the nas to the tv wirelessly , but my tv doesn't seem to see the nas , but it was visible on the wifi from the isp router. i changed on the deco network and cant even acces my nas