rule 1
205H support is 4.2 onwards
rule 2
all IAPs in a cluster must run the same software.
If DNS server is configured and able to resolve device.arubanetworks.com and reach it. Also pool.ntp.org is reachable, then the IAP running X code, trying to join and existing cluster running code Y, should upgrade itself toY to join the cluster, by downloading the code from aruba website over internet.
If you can get the above accomplished. then put the IAP 205H in a separate VLAN, to have it in a separate cluster, then upgrade it the software existing cluster is running. However in your case the cluster itself has to be upgraded. I suggest upgrade it to 4.2.3 and then try to get 205H joined.