Originally Posted by NewbieRunner

Yes, a stopover of 19 hours is allowed as long as you remain in transit area. From your post in another forum you appear to be considering a stay in the transit hotel in FRA.

Your baggage can be checked through if you are connecting from LH to LH but there was a report that baggage could no longer be checked through on an overnight (more than 10 hours) transfer from LX (and possibly LH) to UA. Check bags overnight on connection to UA?

Can you change you FRA - USA flight from UA to LH? You may want to ask about baggage rules in the UA or LH forum.

P.S. This post seems to confirm UA’s policy. It seems to apply both for single and separate tickets.
https://www.flyertalk.com/forum/31062241-post786.html