If IRROP on the day of traveling you contact UA to try to get this solved. UA is in charge of the ticket when less than 24h before traveling. Otherwise you need to sort it out with the ticket issuer. LM can't rebook you unless award availability is present.

BUT UA can refuse to do anything for you and encourage you to travel to IAD as it is not their flight that is disrupted and then refer you to contact LH for rebooking from IAD. That is not a good solution for you as it will be impossible to get in contact with LH for rebooking.