For international connection (DOM-INTL) with United being international and domestic being partner airline, the app doesn’t allow to checkin, neither allows selecting premium seat (it’s T-24 for united flight as well).
(Got Silver status so have to wait till T-24 for complimentary E+)

So wondering if phone customer service can change the seat (don’t want to wait till I get to airport and lose good E+ seat options)?

(Could checkin to domestic flight on partner airline site however that doesn’t allow to change united seat and potentially it may not recognize the status to determine E+ is now available.)