@Akari 

Hi, welcome to the community.

Do you have another router in the network except for Deco M9 Plus?

You could check it by comparing the IP address on https://whatismyipaddress.com/ and the IPV4 address on Deco APP>More>Internet connection.

If they are different, you might have two routers/DHCP servers in one network. So when the UPNP is enabled on Deco, you might need to check the UPNP settings on the first router.

I see you mentioned trying Access Point mode on Deco, the NAS on a static IP address is no longer accessible. I think it may be related to the IP settings on the NAS. It is used to be in the LAN IP range of Deco but after Deco is in AP mode, it also received IP address from the front ISP router, you might need to change the static IP address on the NAS to be within ISP router's DHCP range.

Thank you very much and best regards.