I can't remount /system as rw. I get an error message: mount: Read-only file system

I'm using a rooted Huawei G7 and i'm trying to install BusyBox, but I keep getting an error. I'm 100% sure this error is because there is some weird write protection on /system. I know this because every time I try to change it to RW in ES File Explorer it changes back to RO when I close the app. I also tried some terminal commands but with no luck so far. EDIT: Here is the full log: u0_a161@hwG7-L01:/ $ su root@hwG7-L01:/ # mount … /dev/block/bootdevice/by-name/system /system ext4 ro,seclabel,relatime,data=ordered 0 0 … root@hwG7-L01:/ # mount -o rw,remount /system mount: Read-only file system 255|root@hwG7-L01:/ # mount … /dev/block/bootdevice/by-name/system /system ext4 ro,seclabel,relatime,data=ordered 0 0 … EDIT: When I try the mount command, I always get the error Read-only file system. I don't know why, but could it be possible to put /system in RW mode from the recovery menu?