The command 'adb root' works, but 'adb remount' results in "Operation not permitted" message
Some phones just don't allow remounting via adb remount.

Mount as RW:

adb shell su -c "mount -o rw,remount,rw /system"


Mount as RO:

adb shell su -c "mount -o ro,remount,ro /system"


Also you could use the Adbd Insecure app, but it's not free.