I have managed to set up and use UPI using my ICICI app .
I have a NRE/ NRO account with OCI with ICICI bank and used a UK mobile number

Also use your own bank app ( in my case ICiCI ) to set up UPI and pay with the banks app .. ICICI app has a UPI option now .

Remember to only set up UPI with NRO account and not with NRE account ..

As far as I know for the set up process you have to have the SIM of the overseas mobile number registered with icici inside the phone for this to work . I don't know if this is required for ongoing use..

Set this up in late March/ early April and used it with9ut any issues in July and August on my india visits.