Admin Panel and Login URL Error

Hello,
I had completely reset my website a few days ago and then activated the plugins, but something wrong happened. The login URL that I set instead of the default one was not working, moreover, it was giving a 404 error. After that, I found that the default link wasn’t working out either. I was just able to access the homepage as a normal visitor so the problem was with the admin panel. I deactivated the plugins from Cpanel somehow and accessed my website. But, the custom login URL in WPS settings wasn’t working still.
I was configuring the iThemes security plugin. The interesting point starts now.
In the Hide Login Backend configuration, a warning popped out, saying that I must change the “plain” mode to something else in permalinks. When I did so, changing it to “post”, my access to the admin panel was lost again, and it gave redirecting errors. I fixed this issue by deactivating the plugins again but now I’m just not sure about what to do. It has never happened before. Every plugin was cooperating well with each other. What exactly the problem mighty be?