Perusing
https://www.icomamerica.com/en/download ... cument=545
Page 88 (emphasis mine):
You can store repeater information for quick and simple
communication in up to 500 repeater memory channels
(Repeater list) in up to 10 Groups. Programming
the repeater list is required for DR mode operation
Note: if the 9100 is similar to my Kenwood -- to use a hotspot you MUST be in DR mode, not DV mode. DV mode is basically direct (radio to radio) or local repeater only, and does not transmit the information needed to have calls gatewayed to other systems. (Possibly Icom lets one set everything and somehow figure out which mode to use, but DR mode with a defined "repeater" for the hotspot seems simpler)
Page 89-91 cover how to add your hotspot as a "repeater".
Page 99 covers using that defined repeater.
The manual doesn't have anything specific for reflectors, so you'd have to manually dial in the correct link/unlink commands in the UR field (once you get the "repeater" itself working). Kenwood implemented a "destination select" menu with choices for local, gateway, individual, reflector, where the latter gives entries for link (brings up menu to spell out reflector), unlink, info, an "use reflector" (sets CQCQCQ and proper repeater mode).