The MR30H is a little different. I had to create a separate ID for each SSID I wanted to add to the physical ports on it. Then under Wireless -> SSIDs you'll have the option to assign that SSID as "wired". It's only there when a MR30H is assigned to the network so it's easy to miss. Aftyer that you have to assign each SSID to the port. From there you'd setup the splash page the way you would any other SSID, but on the new one instead.
Another option for the splash page would be to assign the VLAN splash page on the MX if you use one. Security & SD WAN has a menu item simply called Splash Page under it.