I recently installed open wrt on a Raspberry Pi 3 B+ it's updated it's updated and I have EasyTether installed, so I can use my smartphone as the internet connection through the USB. I have added an interface for it and it sees when it's connected. My trouble is I want to use the existing ethernet port on the Raspberry Pi to connect to a routers wan port. Any help
that will work... although its wiser to initially test it with a simpler wired client if you are inexperienced with networking.
Just to get a clue what is involved:
Tried did not work
why not? 
Its somthing simple with the ip addresses because now now I have internet on wifi from the pi only but when I try the lan no connection and when I disable the lan the wifi goes out
OpenWrt on the Pi is a router. You can connect dumb switches and APs to the Ethernet port (in its conventional role as the LAN) to expand the network. It isn't necessary to use two routers that just makes it more complicated.
If you use two routers the IP range used on the WAN port of the second router (the Pi's LAN) needs to be different from the second router's LAN, which also often defaults to 192.168.1.1
This is a way to simple. The link I gave you is not running OpenWrt on RPi its running Raspbian. So things are different but similar. The guy describing things pretty good even with some knowledge about networking in general. You cannot adopt it 1:1. You have to adjust it to your needs.
I would start here. So RPi on default 192.168.1.1 and your router in back with like 192.168.2.1. Don't forget to set proper routes if you want to reach downstream/upstream network and adjust the firewall if you want to reach LuCi web interface from each side.
If you want it simpler go with: