After leaving it alone for a few hours it's now working. I'm not sure why it didn't work this morning. Normally I'd blame the VPN server or a typo during configuration but I was able to connect at will with my laptop and Gemini at the time, and I didn't edit the config in between.
The Cosmo does struggle to maintain its wifi connection more than the Gemini does, and I did reboot the Cosmo at some point. Maybe the flaky connection this morning interfered, or the reboot fixed something. I'll post again if it turns out to be unstable but it's been pretty good since I finally managed to connect.