GL-AR300M port forwarding NOT saved and applied

Hello, in my GL-AR300M I try to forward a port, but when I click “save and apply” nothing is saved. Could someone suggest sth? (VPN is enabled)

Can’t it work, or just you fail to add a port forwarding rule.

Failed to add a port forwarding rule. It says “configuration applied” for a few seconds, but then nothing is added.

Could you please upgrade to the latest testing firmware? We had added port forwarding on our webui. Download from here, please untick “keep settings” when upgrading.

How can I downgrade to 2.26 in case something goes wrong? which file do I have to install?

Something goes wrong as in?
Flashing firmware, upgrade and downgrade, is done through the Web UI.