My experience this summer was to call UA and ask to help (016 ticket, Business, LH Codeshare)
It took a while but she got a supervisor and United took control of the ticket and got my on one of their planes. Call took about an hour so it’s possible but probably not easy.
LH paid the EC/241 comp after about a month delay.
Everyone should experience FRA and LH once in a while. It will help you to appreciate the relative kindness of EWR customer service and the practically alien advanced technology of the UA app.