Do you have the UA PNR? UA may recognize the LH PNR.
DO you have that ticket number, probably starts with 220 and another 9 or 10 digits?
With either you should be able to go to united.com if any flight is UA operator and with MyTrips pull up the reservation.
Or
call or chat with UA
UA provides free advance seat assignments as long as it is not a Basic Economy / Economy Light reservation.