Please read https://help.emteria.com/kb/devices-rpi-3#custom-screen-resolution more carefully. It doesn't say anything about persist.rpi.mode.backup property.
There is no universal way to change the resolution, but there are two basic approaches:
1) If the resolution setting is enabled after a clean install, use it. If you enter a non-supported resolution, re-flash and start again.
2) If the resolution setting is disabled after a clean install, find out why. Probably it is because your screen doesn't support anything else.
If you know the reasons and you are sure what you are doing, you can edit those files manually.
