They have a nice web app, but you must use their mobile app to login on the web version. The app takes a video of a QR code on the web page during login. Web login completes as soon as the mobile app notifies the server. There's no 2FA code to enter, and no alternative.

I asked them about this, by phone call, when my phone screen broke and I urgently needed to make a transaction. Surely there as an alternative? Or could I do the transaction by phone call?

They told me that indeed there is no other option. Despite having phone customer support, they had no phone or web banking service at all which could be used without a registered mobile device. The only phone service they could perform was to register a new mobile device, which I didn't have. I had a tablet, but it was too old.

So I had no good choice. The Android phone I'm using right now was bought in a hurry just so I could be allowed to make a bank transaction.

It wasn't my first choice of phone. I didn't have time to investigate alternative devices, let alone weigh up open alternatives. I ended up buying a mid-range device under pressure that seemed ok and was available in a store without waiting. (It was a brand new Samsung, and despite the IP rating it got water damaged and stopped working entirely after a few splashes a year or so later, but I was able to get it repaired.)

There is also an alternative for now, but nothing as simple as SMS or authenticator app. They give you a special credit card shaped card with a card reader that you can use to authenticate with using your PIN, which is mostly considered legacy now with the bank app. It's also not realistic to be carrying this thing around everywhere either as it's bigger than my phone.

There is also a national ID app that is used everywhere that I'm worried will stop working on GrapheneOS... Because without it I won't even be able to access online government services like healthcare, taxes, etc.