Is it possible to invest with fiat currency?
I think it's a yes. But you want to make sure you can visit their website and read their whitepaper and also join their telegram group. here the link of their telegram
https://t.me/ESRWALLET. All of your question will surely answered since the dev and participants of ESR are active.
ESR Wallet telegram is great, many devs/participants are always online there. So that can answer all your question about internal and external issue about ESR Wallet. Don't hesitate to join and get some freebies as well.