Hi, after 2 days of intense work, I have successfully done the job 
I can help you if you want. very important, I flashed the openwrt 24.10 firmware as maybe the stock firmware is buggy. Other important details: after some save & apply, the router should be rebooted to get the new config.
this post also was very important: Flint 2 (GL-MT6000) VLAN Setup
last thing: the uplink port should be a trunk, in my case is connected to a pfsense firewall and this last one manage the DNS, DHCP, etc so my flint 2 is a "dumb" router in AP mode.
let me know if you need some guidance
*I'm missing the wifi separated SSID for one of the 2 VLAN that I configured, so at the end I have 3 ports and wifi in my main untagged LAN, one port in VLAN 10 and other port in VLAN 20.
btw in my case I had a lot of dns leaks with a giant unique LAN and that's why I decided to use VLANs.