There’s a multitude of things which can go wrong or interfere with a remote access setup. Not all of them are under your control or immediately visible/transparent to you.
Sometimes an ISP changes how they handle your network access or there’s a change with an automatically installed fix for your router.
Solutions might not always available „in layman’s terms“.
To get a better understanding, you might need to share some more details. You could e.g. work through this troubleshooting guide. If that doesn’t help you to fix it, the output/findings of the individual steps should shed some light on what’s going on in your particular setup.
Edit:
While your particular router isn’t listed, you might be able to check/compare if any of the DSL-3xxx or DSL-5xx models have a similar administration and apply the description
