In the default configuration of the hAP ac2, ether1 is your WAN port (for the uplink) , all the rest are LAN ports bridged together.
You need to modify the default config so that ether1 and WLAN2 are switched in the configuration.
Connect via ether2-3-4-5 (or WLAN1 if configured already).
Make the changes in menu "interface" , "bridge" and "IP"
interface: change interface list (ether1 in WAN list) to (WLAN2 in WAN list)
bridge ; change ports: change WLAN2 to ether1
IP : change DHCP client : change ether1 to WLAN2
Now it's time to setup WLAN2 as "station" mode. Use "wireless" menu. First make a "security profile" that has the settings for the authentication to the ISP wifi. (not always easy to find how) Then in WLAN2 set that security profile and frequency mode to "regulatory domain" and country, and installation = "any". Then "Scan..." the environment. Connect to the ISP AP in the scan list.
Should be it. (Unless your ISP further requires an extra user login like PPPoE etc)