If you are unable to set up Singpass app using your existing 2FA modes, you can request for a PIN mailer to be sent to your mailing address that is registered with the Immigration and Checkpoints Authority of Singapore (ICA).
Follow these steps:
Download Singpass app from the official app stores.
Open the app and log in with your Singpass ID and Password.
At the screen prompting for an OTP (see sample below), scroll down and click on the hyperlink in the sentence 'If you are not able to complete setting up Singpass app with your existing 2FA, click here for alternative options.'
Select 'Mail a PIN to my address'.
A PIN mailer will be sent to your registered mailing address within 10 working days (subject to overseas postal services and delays). Follow the instructions in the PIN mailer to complete the app setup, and you're done!
Alternatively, you can set up Singpass app using Face Verification.