First, get the latest 7731 firmware from the Support Portal Downloads page and
upgrade. Upgrade the far side 7731(s), whether RB or an NRBs, first.
Second, and this might be key to improving your performance, be sure you have
the correct Distance setting on both side 7731s. From Configuration::Wireless,
click Edit Advanced Settings button, for the screen with Distance value setting.
Your 75 meters means use minimum 1KM setting.
Third, use the Channel Optimizer from the RB Status::Wireless page.
This is an intrusive test that will flood the link in both directions with as many
packets as possible in both directions, on every available 5G channel to determine
the best bi-directional throughput one to use.
If you experience further problems, use the Channel Optimizer again, but that
might mean there could be an intermittent source of interference that sometimes
affects your link. I hope this is helpful.