See this post
https://forum.openwrt.org/t/problem-installing-on-linksys-ea7500v2/116253/9
I have read in another post if OpenWrt fails to acquire an IP address on its WAN port using DHCP for EA6350 v4, it may revert back to booting OpenWrt. Is the WAN port of your router connected to your LAN during OpenWrt installation?