Prepared the AXP90 today night.
The method of installing an NF-A9 using the original mounting brackets is, as others already suggested, is by bending the brackets at the "V"-corner using a set of pliers, until they fit both the top of the fan and the lowest part of the CPU cooler.
Other option: Just order another Thermalright CPU cooler with 92x25 mm fan, and replace it
cu, w0lf.
The method of installing an NF-A9 using the original mounting brackets is, as others already suggested, is by bending the brackets at the "V"-corner using a set of pliers, until they fit both the top of the fan and the lowest part of the CPU cooler.
Other option: Just order another Thermalright CPU cooler with 92x25 mm fan, and replace it
cu, w0lf.