There is no wiring. Just a USB connection.
I discount a coding error since known-good code works on the second MKR and not the original one.
The dataflow from an Uno, through the hub, and to the MKR was numerous trials of 1020 215-byte messages over 4.54 minutes and of 4080 55-byte messages over 10.88 minutes.
Have done similar with two Unos and never had this issue. LoRa transmission back then was with the Dragino transceiver shield that they no longer manufacture.