@TechnoMonster 

Here is a FAQ with screenshot steps: https://www.tp-link.com/us/support/faq/1797/

@Tony  - thank you.

I'd seen this page last night when trying to set this up.  I tried 5 or 6 variations and none worked for us.  

We had this all working with the standard Comcast/Xfinity Gateway modem/router and had the ports all set up there fine.

But when we added the TP-Link Deco M5, that's where we didn't understand how to set up the port forwarding, and still don't.

If anyone has gotten this to work with a Synology NAS, I'd sure appreciate you weighing in!

Thank you.

@TechnoMonster 

Anyone else able to offer help please? 

@TechnoMonster 

If the Deco M5 is sitting behind your ISP modem/router, then you need to do double port forwarding.

You mentioned doing the port forwarding in the modem/router originally for the Synology. What you need to do now is to change the port forwarding IP to whatever the Deco M5 IP address is that it got from the modem/gateway. You then log into the Deco, and do the port forwarding for the Synology NAS.

Another alternative is to set the Deco M5 as access point mode (https://www.tp-link.com/us/support/faq/2399), here is how you would do that: https://www.tp-link.com/us/support/faq/1842/