I thought that this missing/blank page was due to the fact I had been using Wi-Fi client mode in Luci, which does not correspond to any of the 4 modes in the Network mode settings (Router, Access Point, Extender, WDS).
Not at the moment, I'll have to ask the guys that set it up like so whether I'm allowed to.
But does the repeater mode allow to use the Wi-Fi client in the GL GUI? I didn't find the Wi-Fi client mode in the documentation, although some posts like this one seem to suggest that this is possible...