You likely restored a compromised backup because the backdoor(s) were already laying there. Or you restored to a theme/plugin with a vulnerability and had it qu

You likely restored a compromised backup because the backdoor(s) were already laying there. Or you restored to a theme/plugin with a vulnerability and had it quickly exploited again. There is some lessons to be learned from your way of trying to fix it. Suggesting not to use a software that is in its core pretty stable and safe, is not one of them.