Thanks @alexpl but that’s no solution. I’m not trying to connect to a remote machine from Boxes. I’m trying to connect to a VM booted with Boxes from outside of it. I ran a service in the box but I need to access it from my main computer, and it simply doesn’t work.
Oh! It turns out I had still installed the flatpak version, and then from the dash I only had a single GNOME Boxes icon that opened the flatpak one, which has this known problem.
All I had to do is:
Open GNOME Software.
Search GNOME Boxes.
Uninstall the flathub version.
Reboot.
Now I can use the GNOME Boxes I installed with rpm-ostree and these do have a working network bridge and can be pinged from the host machine.