Here are one user’s threads on the topic:
https://forum.peplink.com/t/kind-of-says-it-all-speedfusion-doesnt-bond-for-increased-bandwidth/
https://forum.peplink.com/t/lte-speedfusion-fail/
https://forum.peplink.com/t/fusionhub-solo-bandwidth-observation/
and another user’s
https://forum.peplink.com/t/lte-speedfusion-is-slower-than-the-fastest-link-in-the-bonding/
In summary, in those cases it seemed to be a hardware limitation, where CPU/hardware was getting maxed out. They weren’t on the 310 5G though could be good to reference and gain familiarity.