The IP address is a fixed one? Did you change IP settings(IP address, default gateway, DNS server) after you use the new router?
You use router's WAN IP and port to access remotely? WAN IP address is static or not?

Vivian wrote

The IP address is a fixed one? Did you change IP settings(IP address, default gateway, DNS server) after you use the new router?
You use router's WAN IP and port to access remotely? WAN IP address is static or not?

Hi Vivian,
I am in correspondance with TP-Link support. Many questions have been asked, including your ones and many more beside.
This is not an easy fix, but to be fair the the Support team, they are at least giving this one a good crack.

Yes I have static IP's assigned (for most of my devices).
The default gateway is the correct/unchanged.
The DNS servers for the clients point the router, and the router gets these from Talkalk - as it should be.
I have also tried the Google DNS servers (8.8.8.8/8.8.4.4) - but this made no difference.
I have, on request from TP-Link support admins, sent a number of screen shots showing my setup, and also sent evidence to show how the port is open for outbound traffic, but closed for inbound. That's the main issue.
Note - occasional the port is opened by the router, but after a few minutes it closes - which is odd.
As a consequence I've even tried temporarily adding an exception in to the Firewall settings, in case the firewall was getting in the way.
I have tried selecting a different port - no joy.
None of these work.
Yet when I plug my old Huawei VDSL2 router (with its unstable wifi) back in place of the TD-W9980, the previously closed inbound port is suddenly open.
So this is not an issue with what I have configured it seems, its an issue with the TD-W9980.

In some case, it might be a limitation on ISP end. Did you try to contact your ISP?
Any anti-virus software or firewall running?

This is not an issue on the ISP end.
As I have said this works fine with the old router. The only Antivirus/firewall I have running are what I always have - in the case of windows clients is standard defender.
There is no special additional/3rd party stuff running on the router. its pretty much out of the box.
Again - I say it works fine with the old Talktalk ISP router.

So here's the latest update. I had issues with inbound and outbound issues on one specific port.
After many, many emails passing between me and their China based tech support I had got nowhere.
After saying I was going to hand the router back I was told this was put through to "vipsupport".

I got an email the next morning from this vipsupport guy who said that he had checked my port and it was fine.
After a week or so of looking at this and it not working - suddenly it was open both inbound and outbound.
Really odd - it was like magic or something. And bear in mind I had not given him remote access or anything.
He sprinkles pixie dust on his keyboard and sha-bang! port forwarding works OK all of a sudden. Head scratching time.

So I ran this for a few days, and then I made a real big mistake. I thought it was safe to add another port to the Forwarding-> Virtual Servers list
Boy was that a mistake. I didn't have pixie dust available, none available at my local Tescos - all out.
So of course it did not work - and not for just my new port, the other working port forward also broke.

Its making me think in a cynical kind of way that they 'vipsupport' has some back door entry to our routers and has made a tweak which I have undone by adding another entry. But hey, that just me being cynical.
It not like TP-LINk routers have ever been open for backdoor attack in the past is it. (there's a number in here if you want to be really paranoid ;) http://routersecurity.org/bugs.php )

What I'd really like is if others here with with TD-9980 routers running the latest firmware could check if they can get port forwarding to work inbound and outbound. You can check if your nominated port is open via the canyouseeme. site.
Appreciate any efforts to check this.