In order to connect to your 3G/4G router over the Internet you will need to know the PUBLIC WAN IP address of the router - however this is not always as easy as it sounds.
This is the easiest way to connect to your router and connected devices because the router is allocated a FIXED IP ADDRESS from your mobile service provider - in most cases fixed IP SIM card services are only available from an MVNO (Mobile Virtual network Operator) who have interconnect services with the main mobile networks and offer the Fixed IP element as part of their service.
THIS MEANS THAT YOU WILL NOT NORMALLY GET A FIXED IP ADRESS IF YOU OBTAIN YOUR SIM CARD DIRECTLY FROM THE FOUR MAIN MOBILE NETWORKS IE. VODAFONE, O2, EE & THREE.
1. Public Fixed IP SIM -This means that you can connect to the router and connected devices from any computer connected to the Internet providing a convenient and easy to use service.
2. Private Fixed IP SIM -In order to access your router and connected devices you will first need to create a VPN from your PC/network to your Fixed IP SIM card provider and only when the VPN is established will you be able to access your router/devices.
Many 3G and 4G router users would like to use DYNDNS to access their routers because this means they can take advantage of low cost data plans direct from the main UK mobile networks - however most mobile networks now use CG NAT (Carrier Grade NAT) to provide IP addresses which means that the WAN IP address allocated to the router is a PRIVATE IP address on the mobile operators internal network so the Public IP address that you might see when you visit www.showmyip.co.uk will be one of the mobile networks public IP addresses but the mobile operator will have hundreds of devices using each public IP address which is why you will be unable to connect to your device. You can research this further by searching the internet for "4G CGNAT".
THE MAIN 4G NETWORKS DO NOT PROVIDE SINGLE PUBLIC IP ADDRESSES WHICH MEANS DYNDNS WILL NOT WORK.
Once you realise that you can't use DYNDNS after researching CGNAT you might not want to use a Fixed IP provider and want to explore alternative ways of connecting to your router.
As long as your 3G/4G router has built-in VPN services then you can configure the router as a VPN CLIENT so once it has established a 3G/4G connection it will then connect to a VPN SERVER which could be your router at head office which would enable you to create a WAN (Wide Area Network) - this would enable you to connect to your router and devices over the VPN - this solution might be fine for a couple of remote locations but once you start to have tens or hundreds of remote locations then you would need to ensure that your VPN server is capable of managing the WAN infrastructure.
YOU WILL ALSO NEED TO ENSURE THAT YOUR VPN SERVER CAN BE CONFIGURED TO ACCEPT INBOUND CONNECTIONS WITHOUT A FIXED IP ADDRESS - because your connection will have a different router WAN IP address to the router PUBLIC IP address - (remember the CGNAT).