Wireless controllers offer many options for the RADIUS Called-Station-ID:
You said you are using AP MAC address:SSID which is a perfectly fine. It is your choice depending on how you might want to use this information in your ISE Authentication and Authorization policies. This allows you to match the SSID in your Authorization Policy to provide the appropriate level of access for that wireless service (Guest vs Corporate vs BYOD, etc.) with a rule like:
If you're not seeing any RADIUS traffic in ISE from the Wireless controller, then the problem could also be elsewhere. E.g. if you're WLAN is configured for 802.1X (or MAB or iPSK) then of course you have to specify your RADIUS servers.
1) Are the IP addresses of the RADIUS server correct? Can you ping ISE from the WLC CLI?
2) Are the RADIUS shared secrets matching on WLC and ISE?
3) What does your ISE LiveLog look like? No activity at all? Then I would run a tcpdump on that ISE node to see if anything is coming in.
The Called-Station ID is correct for your needs. The SSID is very useful in Authorization Policies if you need to go down to that granular level.
Clients are not affected by this change made at the RADIUS level (of course as long as the RADIUS flow still works as expected and completes as expected - but the format of individual attributes is not visible to the clients.)