I have contacted TP-Link giving them all the details, including a route trace, and their response was in essence that it was a problem causing by the ISP creating a 'double NAT situation'. They were unable to provide a generic solution or work around to this but said that ISPs have developed their own specific solutions, so I'm hoping that iD mobile have a solution to this.
Best answer by Mohammed
DDNS Over 4G
If you use a standard 4G mobile broadband SIM card in your 4G router then you will be unable to use Dynamic DNS. This is because 4G mobile network providers in the UK use Carrier Grade NAT (CGNAT) which means that your 4G SIM connection is allocated a PRIVATE IP address on the mobile networks private network.
So if you have a PC connected to your 4G Router and visit www.showmyip.co.uk then the IP address you will see will be a public IP address belonging to the mobile network and your SIM card connection will be sitting behind this public IP address along with 250 other devices all with private IP addresses.
Therefore in order to get connected to YOUR device you would need to contact your mobile provider and ask them to remove the other 249 customers sharing that public IP address and port forward the public IP to your private IP – As this is controlled by the MNO and not the MVNO, Three would not support this.
The way around this is to just use a 4G Fixed IP SIM card – there are many reliable fixed IP SIM card providers in the UK that will be able to provide a service to enable easy and reliable connectivity to your 4G Router.
If you don’t want to use a fixed IP SIM provider then your only other option is to use a three mobile SIM card and use the APN of 3internet and configure your router to connect at 3G only. This way, when your router connects to the Three mobile 3G service it will request a public IP address and if there are any available then your connection will be allocated a public IP address and you will be able to use DDNS. However this is not a contracted service from 3mobile and may be stopped at any time and in some cases your connection will not be allocated a public IP address so your DDND will not work.
How to get your router working with a three mobile SIM
Remember this is not a guaranteed solution and you will not always be allocated a PUBLIC IP ADDRESS so DDNS will not always work. A Public IP address on your 3Mobile SIM card is not a contracted part of your 3mobile service and may be withdrawn at any time.
This 3mobile connection will only work when using 3G – if you use the APN of 3internet and connect at 4G then you will only be allocated a private IP address which means that DDNS will not work.
For your convenience here is a quick video showing how to configure a RUT500 3G Router with a standard iD Mobile SIM Card – changing the APN to iD, using http://www.4gsupport.co.uk/ddns-over-4g/ to find out your public IP address, enabling remote admin to confirm that the Public IP address is working and to configure port forwarding.
Hope this helps!