If you are using http, make a NAT rule in your router to ip address of your NAS and port 80.
If using https, make a NAT rule in your router to ip address of your NAS and port 443.
Use DDNS in Synology.
It should work.
To test, take your smartphone, disable WifI and use mobile network. Connect to DDNS.
DDNS wil only work external. Internal use ip address of NAS.