FortiSwitch 548d to replace Cisco 2960 access switch?

I just received a 548d to test with. I would like to configure it as an access switch, plugging into my Cisco core 9300.

On the Cisco side, I would typically configure something like switchport mode trunk switchport trunk native vlan X

On the uplink port to the core. And I’d assign an IP to VLAN X. From that point, I could SSH to the access switch.

I’ve been unsuccessful trying to accomplish the same on the 548d.

I created VLAN X and assigned an IP address to it. Then I assigned VLAN X as the native VLAN for the uplink port. However, I cannot ping the assigned IP address.

I did notice the link light on the 548 is green, but the Cisco side is blinking amber. I’ve tried a couple different STP settings on both sides unsuccessfully.

Am I missing something easy here, or going about the configuration wrong?

Thank you!