The solution and documentation provided by Jozko would be the ideal approach to this problem, but try running a zdebug + drop on the Active firewall while you attempt authentication to the Secondary.
Chances are the Radius access/accept packet might be dropped by the Active firewall since the return traffic would be sent to the Cluster VIP.
In some of the cases I worked with, setting the kernel parameter fwha_forw_packet_to_not_active to 1 fixed the issue. But this also depends on the drop logs seen on the Active firewall.