I have a similar issue.
My ISP modem connected to port 0/0 on RAP-155P (only one port on the modem and it can only provide 1 IP address via DHCP).
I use the RAP-155P in Instant mode to act as a home wireless router also with a 24ports PoE switch connected to port 0/4.
I tried to connect an IAP-105 to the 24 port switch and to one of the other available port on the RAP-155 without success. The IAP-105 come up as it's own instance of Virtual Controller and doesn'T show up as an AP in the RAP-155 Virtual Controller.
IAP-105 have been factory reset.
Both IAP-105 and RAP-155P are to the same code level (6.4.2.6-4.1.3.0_54915).
Both are RW (Rest-of-World) version so regulatory compliance should not be the issue here.
My Local SE suggested me to tag the port that connect to ISP as an uplink port. I did it from the wired port config and also from the System, advanced option, uplink tab under Management I selected Ethernet as Enforce uplink.
Still doesn't work...
Any idea of something I miss or did wrong?