iPhone: Settings → Cellular → the travel line → Network Selection → turn Automatic off, then pick a network from the list.
Android: the SIM’s settings hold the same choice, usually as "Choose network automatically" with a list underneath.
Try the operators your plan lists for that destination. The plan page names the networks it uses where the network publishes them.
Switch Automatic back on when you move to another city or another country, or the phone will keep hunting for a network that is not there.