Remote Access ONLY working through VPN on remote iOS devices

Server Version#: 1.41.5.9626 (on Synology NAS)
Player Version#: 2025.28.0 (iPhone and iPad)

On a recent trip abroad I could only connect to Plex (on my Synology NAS at home) if I had a VPN running on my iPad. If the VPN was off then it would not connect. I could not connect on the iPhone either and I don’t have a VPN on that.

All the topics I have searched here appear to be problems connecting when a VPN is active, but my problem is the opposite.

There is no VPN running on the Synology.

Logging in to the Plex server, Remote Access is working with the green message ‘Fully accessible outside your network’.

Does anyone have any ideas what’s going on or what I need to check? Thanks in advance.