The first case, I thought perhaps, I just overtightened the connector and put too much pressure on the case. So, I ordered a second one and was extremely careful while drilling and tightening the SMA connector and it still cracked. When I drilled the holes, I started with a 1/32" bit and increased in 5 drill sizes to the final hole size as to limit the stress on the plastic. They were all brand new drill bits.
What surprises me is the response my distributor received from Mikrotik while trying to find replacement cases. Apparently, installing the external SMA connectors (Mikrotik's own product) will void your warranty?! I really wish I would have known that considering they promote the fact that you can use their external antenna kit. What makes matters worse is - they won't even sell my distributor a replacement case (I have NO problems paying for replacement cases). So, I now have 2 units that can't be repaired
I've since, just come up with an alternate LTE solution that allows me to use external antennas.