Bricked My Router D7800

I was loading OpenWRT on mt Netgear Nighthawk X4S D7800 but it's bricked. All was going well until near the end and then it frozen. After 5 mins the Netgear page appeared in the browser but the router was dead. Now I can't recover it as the power light is hard on orange and then goes white, it doesn't blink orange which is when to do the PUT in TFTP. Any ideas?
The next question is, I'm keen to use OpenWRT so I can use the split tunneling via Port feature (I think I have to load a package later for that) I may have to buy a new router that powerful enough to do this and has OpenWRT already loaded. Can anyone make a suggestion what is the best?

Many thanks for the info. I'm new to this sort of thing but did follow the instructions including the checksum test. I loaded openwrt-ipq806x-D7800-squashfs-factory. My router says D7800 on the bottom of it. The router doesn't flash orange, it goes from hard orange to white so, I can't seem to load another firmware using that method. I'll probably buy another powerful router with OpenWRT already installed.