Currently I have the Netgear WRT2200 running OpenWRT connected to a Huawei 4G router/modem for WAN access, with this setup I can't get port mapping to work so I want to connect a simple USB 4G modem to the WRT2200's USB port. Any idea how I go about to get that working? What do I need to do in openwrt on the Netgear router?
Have you created an interface for your 4g dongle and assigned the interface to a firewall zone?
No I have no idea how to do that so was hoping for some help here 
Are you using Huawei E8372/E3372?
First you need to install some packages.
opkg update && opkg install kmod-usb-net-rndis kmod-usb-net kmod-usb2 usb-modeswitch kmod-usb-net-cdc-ether
Then login to luci - Network - Interface - Add new interface...
eth1 is the interface name of your 4G dongle.
Firewall Settings - Create / Assign firewall-zone - Assign to 'wan'
Then you can add port forwarding rules.
You also need to set port forwarding or enable DMZ on your 4G dongle. Tutorial here:
https://www.cfos.de/en-us/cfos-personal-net/port-forwarding/huawei-b315s-936.htm
Thanks for your reply! No I'm using a Beelink 4G dongle, once set up (on my laptop) it just plugs in and connects ready to use but I don't know what protocol it is using. Would the procedure be any different?
Most of them should use RNDIS, so it may also work.
Well, I can't connect to any sites, the browser just says "name not resolved", where do I assign a nameserver? Can I use 8.8.8.8?
Any DNS server that your network can reach is OK. Are you sure the interface name of your 4G dongle is right? What does "Luci - Network - Interface" say?
In "Network - DHCP and DNS", you can set DNS server in DNS forwardings
.
You mean WNR2200 v3, right?
By bridging LTE and USB0 I'v got internet access over the dongle and name resolution seems to work now. I'll give it a go setting up dns forwardings.
It does not say v3 on the back, it there another way of determining the version?
My point was rather WRT2200 (not listed as supported) vs. WNR2200 (supported).
Which OpenWrt image have you flashed on your device?
Now to the next question, is it possible (or complicated) to connect an USB WiFi dongle using an USB HUB and get 5GHz network from this router?