Sorry to hear of your issues

Unfortunately flight cancellations are high and there is essentially no spare capacity
Yesterday UA cancelled 3% of its systemwide flights, 6% out of EWR (AA was also 3% systemwide and DL 7%)

Most UA TATL aircraft have PremPlus but not all, so with aircraft changes it may or may not be available
In recover from reduced operations from prior two years, UA has hired a large number of new employees and their training / knowledge levels leave much to desire

Do you have a replacement reservation? If you provide some details, those here can look up more information on the availability of PremPlus.