You cannot learn what are the limitations of your upload bandwidth at ISP side or further in the network. You can use Mikrotik’s queues to actively limit the upload bandwidth of different classes of uplink traffic generated in your LAN (and by the router itself) according to your preferences as it leaves your router. There’s a nice guide on that by @pcunite.
But it is still weird that you’d be getting full 5 Mbit/s on one uplink and zero on the other one, it rather sounds like some mistake in the load balancing… do you use per-connection-classifier or some other strategy to choose an uplink for a connection? The per-connection-classifier may allocate the same uplink to all iperf3 instances due to inappropriate choice of the classifying hash.
If there is really no issue in your configuration, it may also be the ISP cheating.