We must add a second controller for redundancy - high availability.
We need to know if an NX5500 can do this job , and if can adopt the existing AP621s.
There are a couple of issues here.
1 - to cluster two controllers the need to be the same model You can not cluster an RFS6000 and an NX5500
2 - 5.4.3 is extremely out of date. you will need to upgrade, but you be limited due to the AP models you have installed
3- AP621's are obsolete. Would recommend investigating upgrading to a newer model
5.8.4 is the last version released by Zebra before the WiNG portfolio was sold to Extreme, and that version appears to have AP621 support, and it is within the supported versions of the NX5500, so it is within the realm of possibility to replace the controllers first and then roll out newer APs without incurring significant down time, and then upgrade to the latest firmware release.