Success! For some reason my original hotspot configuration (a Jumbospot modem clone on a Rasperry Pi Zero W) couldn't maintain network connections. It'd connect to DMR and YSF servers but time out after a minute. I moved the modem to a Raspberry Pi 3 and it's working perfectly now, and I'm able to change the Brandmeister settings for it.
Thank you, everyone, for your help!
How do I tell Brandmeister I have a hotspot? (New guy)
Re: How do I tell Brandmeister I have a hotspot? (New guy)
Good to see you got it working.