Several things could be going on there.
First, you need to have all 3 antenna connectors on the AP124 hooked up to antennas. Is this the case?
Second, the ANT-19 is a stick omni type antenna with a pigtail. Even if you had three of these hooked up, your performance is likely going to be poor because they all have the same pattern and the same polarity.
With MIMO, it's critical to have differences in pattern and polarity to help the RX side decorrelate individual spatial streams. On our outdoor APs, we sell kits like ANT-2x2-2005 and ANT-2x2-5005 that include two antennas. One is an an H-polarization and one is a V-polarization. They have been designed to have similar patterns with just the polarization difference. This makes a significant difference in measured throughput.
We have seen other customers have issues combining multiple V-pol omnis for mesh. switching antennas usually resolves the issue assuming that there are no other impairments in the environment.
You can test this out with the ANT-17 which has 3 elements with +/-45 degree polarizations. You should be able to maintain a mesh connection successfully out to say 100 meters with two of those pointed at each other. This is a lower gain antenna than we would normally use for a mesh link, but it will help narrow down what you're dealing with.