The MT7621 is a current-production chip that is well supported. It has dual advanced MIPS cores which is quite a bit of CPU power.
So like @sparks said, it really depends on what particular model router you have-- that establishes how to install OpenWrt and the ports and interfaces you will have available.
Make sure your 4G modem works with desktop Linux with an actual test where you confirm you can reach the Internet. Then look at which drivers it uses. These can then be installed on OpenWrt. Testing on a desktop first saves a lot of confusion should troubleshooting be required.
well, i can see theres no light on and no drop down selection for were it may be for the usb driver (for stored files) and no drop down choice for 4 g. i def think it has a wrong version of openwrt if thats possible