Since this is the UX StackExchange, I will answer around the user experience concepts associated with F2A.
Applications have F2A authentication to protect a user from identify theft and monetary theft and the company from legal action for Personally Identifiable Information(PII) being released. Skipping F2A or OTP for verification is simply not an option with the sensitive information the app contains: payment info, PII data. While F2A is not a legal requirement, it is a great tool for fraud prevention.
That said, likely there are portions of the app that can be explored without displaying PII or offering the option to purchase, such as looking at train/flight times, locations available, or pricing. These should be available without the need for any authentication.
This alone would lead to a recommendation that F2A authentication is reserved until the point when a user would be accessing PII or ready to start the payment process.
Ease of Use
Best to make things easier for your users whenever possible and unfortunately, F2A is an extra step. As an extra step it should be reserved for when it is truly necessary and not introduced earlier. If a user can avoid it altogether, their use of the app will be easier.
Another aspect of ease of use is flexibility and options. Give users options for F2A or OTP or using biometrics if it is an option. And as James Coyle mentions, flexibility in payment options is a win for the business and the users: the business can defer security to the payment providers and the users have options to select the payment method they prefer.
Perception & Emotion
This will require some research to understand the best approach. F2A protects users, but there is also the perception of protection to consider. Do users feel uncomfortable with not having been prompted to authenticate right away or did they not even notice/care? How far can the application go without asking for F2A before users start to feel uneasy and ask a test facilitator "Shouldn't the app be asking me for login info?"
Targeted user research will help answer these questions. Combine the perception with the need and ease of use to determine the right time to ask for F2A.