Option 1: Check if UTS App's Book and Travel Paperless is available at the station and book the ticket before the arrival at the station.

Option 2: If not, check QR code bookings. If QR based booking is available, you can get the QR for the source stationfrom the internet or you may request any local RF from the station to take a pic and share. Scan the QR while you are at the platform.
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The advantage of option 2 is that you can book it right on the platform itself.

If UTS isn't available at the station, you may get down at a previous junction or a big station where the train halts for a while and book a UR ticket from that station and get back on your express train. If UTS app works at the junction/big station (which it should), use UTS app. The fare difference wouldn't be much

Vivek