If you need to create a VPN between your 3G / 4G router and your head office then you will sirst of all need to know what the head office VPN capabilities are so you when you call your 3G/4G router provider you can answer simple product selections questions such as What VPN Protocol do you need (IPsec/IKEv2/PPTP/OpenVPN/GRE) and also the details of your head office VPN server.
With this information about your head office VPN server and the VPN type required then your proposed 3G/4G router supplier can make a recommendation of a suitable 3G or 4G router.
This assumes that you are going to configure your 3G/4G router to connect back to your head office because you are using a mobile data SIM directly from the mobile networks that does not give your connection a public IP address so you can’t connect directly to your router and want to use a VPN in order to create a wide area network so you can connected to your remote devices without using the convenience of Fixed IP SIM Cards.
In some instances, remote monitoring equipment manufacturers will state that their equipment can only be connected to over a VPN and in these cases you may want to use a Fixed IP SIM card in conjunction with your 3G/4G router so that the 3G/4G router can act as the VPN server so you can connect to it from any device on the internet and create a VPN so you can access your devices on the LAN.
So it is worth gathering information about your intended use of the VPN along with any other special requirements before deciding which 3G/4G router to use as this will save headaches further down the line when you purchase a router only to find that it does not support the protocol or requirements of your installation.
It is also worth noting that if you are using the 3G/4G router to create a site to site VPN then you will already need to be familiar with the head office VPN server / VPN router configuration because you will need to configure this in addition to your 3G/4G router and your 3G/4G router provider will usually only assist in the configuration of the 3G/4G router and it will be your responsibility to understand your own hardware or get support from your head office VPN server / router supplier to provider VPN assistance with your head office hardware that you have purchased from a different vendor possibly.