The easiest way to handle the situation is to get a new phone where you can download the app.

You can't just switch to a paper pass. You need to make an exchange of passes and this can only be done if the conditions of your pass allow it. 

So first of all you need to check the conditions of your pass . You find the them on the invoice that you got with the confirmation email. The invoice is also under the Order Overview in your Interrail/Eurail account.

If the conditions allow an exchange you first need to buy the new pass and then send both the old and the new pass numbers to Customer Support. You will then get the price of the old pass less 15 EUR refunded. If you bought PLUS you get the full price refunded.

https://www.interrail.eu/en/interrail-passes/ordering-info/refund-exchange-policy

The only way I am aware of is to buy a new paper pass and then apply for an exchange from your existing mobile pass. There will be a 15 euro fee. (ie pay for the new one then get a refund on the old pass.)

This is of course assuming your original pass wasn’t bought in the sale last year, where most weren’t refundable or exchangeable - see your T & Cs in the original e-mail.

In this case you need to talk to customer service via e-mail to see if they can do anything.

Personally I would consider borrowing a suitable phone (I have 2 sons who will have a spare somewhere), or even popping into the local second hand phone shop.

Paper passes take some time to arrive so again check with customer service if your need is imminent.

Be sure to include your travel date in the e-mail header so they can prioritise it.

https://eurail.zendesk.com/hc/en-001/requests/new

Best way buy a refurbished phone! In UK iPhone 6 available widely and it supports the app. Lots of sim only deals to go with it. Never buy a brand new phone it’s a total waste of money and bad for the planet