@Taffy4400 

You can only run Deco mesh in Access Point mode if your main modem/router runs in its default router (or gateway) mode.

Assuming you haven't made any changes in your main modem/router, or rolled them back, here is how I would recommend to proceed:

1. Power off Deco mesh;

2. See if you still have Internet access over WiFi from main modem/router. If you do, everything is set, if not - figure by yourself or with ISP Tech Support how to restore modem/router to its original settings;

3. Connect Synology NAS by Ethernet cable directly to main modem/router. Bring Synology NAS to modem/router if necessary - this is temporary. Configure remote access to NAS. You are now configuring remote access through regular router, Synology forums and Internet search could help with that;

4. After you have configured remote access successfully, power on Deco mesh;

5. Configure Deco mesh to run in Access Point mode: Switching a Deco to Access Point (AP) Mode

6. Turn off main modem/router WiFi, your Deco mesh provides WiFi coverage now;

7. Power off Synology NAS, unplug it from main modem/router, take NAS back to its permanent place, connect to Deco unit, power it on. Remote access to Synology NAS should still work.

Done.

  @Alexandre. Thank you for your help it was helpful. The problem was easily resolved. Anyone with the same problem as me should just call Synology customer service it can be easily resolved.