I didn't realize you were accessing the RPi via KStars running in Windows; I thought you were running KStars directly on the RPi but via a VNC or browser interface from Windows.
I haven't yet set up a remote astrometry solver. As you're running Windows on your workstation, you don't have much choice in the matter, but I wouldn't normally set it up with the remote astrometry server running on the Pi as its processing power is limited compared to workstations. I mean I have it running locally on the Pi, but that's to ensure I have it when I'm in the middle of nowhere.
After an hour or so at this using my desktop machine as the putative astrometry server and a laptop as the client doing astrometry look-ups, all I get are solver failed errors.
I had the indi astrometry driver loaded and enabled on the server profile (with Telescope Simulator and CCD Simulator) and the same on the client, with it connecting to the server's IP address. Everything appears to load up fine on the client machine. From the server machine, I can do a local (i.e. offline) "Resolve and Slew" on an existing FITS but when I try that on the client with "remote" set it just fails.
Is there a guide somewhere detailing exactly what has to be configured for remote astrometry to work, on both the server machine and the client machines?
I eventually got it working... it seems the Remote solve is far more sensitive to insufficient astrometry.net FITS files than a local offline solve on the very same machine...? I'm not joking here: it would regularly solve on the host machine as "offline" but would continue to fail to solve when launched from the laptop in "remote" mode.
So really load up on the astrometry.net FITS on your solving host machine.
The host machine profile needs to have the INDI Astrometry driver loaded and enabled (enable it in the INDI Control Panel).
If the Astrometry driver isn't loaded on the remote host, you get a message on the local machine:
"Cannot set solver to remote. The Ekos equipment profile must include the astrometry Auxiliary driver."
But confusingly, that's not referring to the Ekos equipment profile on the local machine, but rather on the solving host machine. Moreover, you'll get that message if the Solver isn't enabled on the remote host machine, even if the Astrometry driver is loaded. I haven't been able to consistently bring it online from the outset.
Overall this all seems a little bit less useful than I first imagined. Let's suppose you've got a limited-capacity device like an Astroberry at your mount doing direct mount and camera control which you access over VNC/browser, but you've also got a powerful server machine elsewhere on the network. It would seem to make sense to install astrometry.net and its FITS files on the server machine while letting Ekos on the Pi do lookups to that machine, but you can't: the Astrometry driver is tied to the rest of the INDI controllers on the host machine, meaning it has to be on the Astroberry*. Now if instead you've got the Astroberry just running as a barebones INDI server while running Ekos on your desktop machine, you might as well just run the solver on your desktop in 'Offline' mode. It's only if you have Windows and you can't run astrometry.net locally does 'Remote' make any sense.
*You might be able to do this by changing the API url of the solver in the Astrometry.net options and then using the "Online" solver, but you're going to have to configure yourself an astrometry server, if that's even possible.