If you collected statistics in version 5.2, then when you finish upgrading the primary BIG-IQ to version 5.3, you need to upgrade the statistics agent on each BIG-IP device that had statistics enabled before the upgrade.
To properly communicate, BIG-IQ® Centralized Management and managed BIG-IP® devices must be running a compatible version of its framework. If the frameworks are incompatible, BIG-IQ displays a yellow triangle next to the device in the BIG-IP Device inventory.
When you upgrade a BIG-IP device running version 11.5.x to another 11.5.x version, or to an 11.6.x version (for example, from version 11.5.3 to 11.5.4, or from version 11.5.3 to version 11.6.1), you must upgrade the REST framework so BIG-IQ can manage the device.
When you upgrade BIG-IQ from version 5.x to 5.3, you must also upgrade the REST framework for all BIG-IP devices (currently in the BIG-IP Device inventory) running a version prior to 12.0.0.