Getting Login Portals to work from anywhere

Firstly I would like to thank you for your excellent youtube channel. Quite frankly I think I would not have been able to set up and maintain my synology nas without it.
I have a suggestion for a future video as I am stumped about Login portals. You don’t have a video directly about this and I have tried to follow other peoples tutorials and advice but with no success.
My issue is that I want to use the login portal so that I can give my family access to synology photos and synology notes from outside my network. Currently they have access via Iphone apps but I want them to have computer access. My requirements are not very demanding

  1. I want a URL they can click on to take them directly to synology photos
  2. Nice to have (but not essential) I would like to be able to use this link from inside or outside of my network (I can live with 2 different URLs though if thats not possible).

I have been able to get access to the NAS using quickconnect.
I have also set up DDNS with help from one of your tutorials
I have port forwarded 5000/5001
But when I try to set up an application portal It works from inside my network but not externally
I tried lots of things but nothing has worked. There are lots of settings and tick boxes and I feel sure I have missed some crucial step.
I have not seen any good videos that make sense on this. Everyone just glosses over the difference between accessing internally and from outside the network. Mostly people just assume this feature is for inside your network and when they talk about external access they are vague about DDNS and the specific port forwarding settings.
Anyhow, Im not expecting any sort of direct response to this note (you aren’t an unpaid synology support agent) but I just thought this would be a good topic for a video as you obviously know how to do this.
Another topic that would interest me is how to add a domain to your nas. I own 2 domains hosted at godaddy but wouldn’t have the first clue how to use them on my NAS.
Thanks again for all your incredible content. On behalf of all us Synology amateurs, I dont think we could own a NAS without your content.

Cheers

Stuart in the UK