Without the bank connection have you been manually importing your transactions on CSV or creating manually? In regards to which category to use I wouldn't be able to advise due to not being a trained accountant but another member of the Community well be able to help.
I imported the csv file. I did notice though that importing a csv file with bank interest as income it works okay. You just cannot enter it manually. Why? I need to enter it manually as its interest earned on a NS&I (also not supported) account and I want to log all interest earned.
I solved my problem by manually producing a csv file and importing it?