Port Forwarding / FTP not working after upgrade to Fibre

[#1]

I upgraded to fibre in the past 2 weeks and since my ftp server "Serv U" has become inaccessible to external clients.
I tried to configure the supplied router (Technicolour) and then replaced it with an Asus RT-AC56U to forward ports 20, and 50000-50004 without success. But port 21 forwards successfully.
I have a fixed IP address and testing access to these ports  (both from the inside and outside) I can see that port 21 is open but port 20 and 50000-50004 are all closed.
Everything was working perfectly before the upgrade and nothing was changed on the FTP server. The server is still accessible internally but when trying to access in externally I get a socket error. I think it is 1005 error
Plusnet say all ports are open and it must be a hardware isssue.
I think I tried everything, can anyone help/advise.
Thanks in advance,

[#2]

Quote

I tried to configure the supplied router (Technicolour) and then replaced it with an Asus RT-AC56U to forward ports 20, and 50000-50004 without success. But port 21 forwards successfully.

Hi, just want to check do you mean replacing the Technicolor resolved this or is it still the same with the Asus in place?

[#3]

Check the Plusnet Firerwall setting in the Member Centre:
https://portal.plus.net/my.html?action=firewall&firewallPage=basic
It may have set itself to Low when you changed your account type.  If you reset it to Off you'll need to drop your connection and reconnect for the change to become active.
There should be no need to forward port 20 by the way...

[#4]

Quote from: _Adam_Walker_

Quote I tried to configure the supplied router (Technicolour) and then replaced it with an Asus RT-AC56U to forward ports 20, and 50000-50004 without success. But port 21 forwards successfully. Hi, just want to check do you mean replacing the Technicolor resolved this or is it still the same with the Asus in place?

Still having the same problem with the Asus in place.

Quote from: pwatson

Check the Plusnet Firerwall setting in the Member Centre: https://portal.plus.net/my.html?action=firewall&firewallPage=basic It may have set itself to Low when you changed your account type.  If you reset it to Off you'll need to drop your connection and reconnect for the change to become active. There should be no need to forward port 20 by the way...

I checked my firewall and it was already off.