When I forward one port, all ports open on MT7620 17.01.4


I'm quite new to Lede routers but have some networking experience.
I've installed LEDE 17.04.1 on a generic chinese MT7620A chipset router which worked amazingly stable with Padavan firmware.
It is connected to cable modem and works well.
I've tried to open only port 5900 for vnc using LUCI

Network-Firewall-Port forwards

Name 5900
Protocol TCP
External zone WAN
External port (Blank)
Internal zone LAN
Internal IP address
Internal port 5900

For some reason is opens ALL THE PORTS (Like DMZ). When I disable this port it blocks them all again.
I would appreciate your advice.

Thank you !

External port (Blank)

Above matches ALL THE PORTS.

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Thank you SO MUCH for your help mikma !