This is SOP when traveling on a different airline first. You can not access the United check-in tool, including with partners, until after you've boarded the last non-UA segment. It's not usually as big a deal when its a partner who can at least check you in from their own system.

Had this issue a while back, when connecting from CX to UA on the same ticket at HKG. CX said they couldn't check my family in on the UA flight, and had to deal with it at transfer desk at HKG. Was a short connection too, and we arrived late, so that was close....literally had to run through HKG to Ua transfer desk, then run back to the new gate - we were the last party on that flight....feel like we are lucky we even made i.