E750v2 very weak 5GHz wifi

Hello, I have tried using my custom OpenWRT build on the E750v2 but the 5GHz WiFi is very weak after 2 feet it will disconnect and very low signal strength.

I then tried the GL firmware it is working correctly. I have used the same build on the E750 first gen and 5GHz was working fine. Wonder if there something I need to enable in the custom build.

Why you require to use the custom OpenWRT firmware?