Sadly, the LAP521 (500 series) will not work with other WLC Models like the 2106 OR 4400 Series only with WLC526.
Q. Can the Cisco Mobility Express Solution be used with the Cisco Unified Wireless Network solution?
A. Access points from the Cisco Mobility Express Solution can associate only with controllers within this solution. The Cisco SBCS was built with the needs of SMB customers in mind. Because these needs are different from those of enterprise customers (for example, simplified network management), the two solutions were intentionally designed differently.
Q. Which access point products are included in the Cisco Mobility Express Solution?
A. Two Cisco 500 Series Wireless Express Access Point products are in the Cisco Mobility Express Solution; their part numbers follow:
To add to this though, I'm thinking the Express 500 Access Points probably do not have enough memory/flash to run the LWAPP code needed for non-express controllers.
I know the 1000 Series AP(and 1500 Series Mesh AP) are not supported in 5.X and it is my understanding that these APs are not "powerful" enough to run this code. I'm guessing it is a similar case with the Express APs. (That, and Cisco probably doesn't want thier Corporate/Enterprise customers putting in Express gear....)