Hi.
Read this.
Ok it's for the EA7300 and you have an EA7500, but the issue is probably the same.
The goal is to flash stock firmware on partition B from partition A. At next boot, it will be on partition B. Flash OpenWrt, it will be on partition A. Reboot, you should be on OpenWrt. Reboot several times. At the 3rd, you should not boot on OpenWrt, but into the stock firmware. This means that you have the same bug that on the the EA7300. The solution should work as both devices are almost identical.
Complete infos in the link above, and on this thread Can not install OpenWrt on Linksys EA7300 v2 - #11 by thosmos