I installed openwrt on my linksys e8450 back in Feb 2024, I believe it is 23.5.0. Two weeks ago I tried to reboot from the web interface and result was no lights on router. I had to order a usb ttl adapter which arrived this week. After a few days of trying to do the serial recovery I thought I was finally successful. I tried the preloader and ubi for 23.5.3. From the U-boot menu I did option 8 as suggested. Files transferred and completed the 4th writing as suggested. Did a reboot, but router still doesn't want to power up.
The BL2 version on that router is unfortunately vulnerable to OKD, and the failure to load error suggests that's exactly the issue you're facing now. If you're running firmware 23.5.x, you want to install the preloader from OpenWRT 22.03.7, not 23.5.x. Sadly, the preloader version released with 23.5.x has the issue.
This looks like simple OKD, so all you should need to do would be to replace the preloader with the one listed for the Linksys E8450 UBI model, just with version 22.03.7. As of today, it links to the below file:
