DDNS (Dynamic Domain Name System) 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.
The way around this is to just use a Fixed IP SIM card.