Is it permissible to continue paying the installments for the house to a Riba-based bank, or must the house be returned? And what is the ruling on buying a house through Riba-based banks in non-Islamic countries when no Islamic banks are available?

You are not required to return the apartment to the bank or sell it to pay off the remaining installments, and you are excused due to your ignorance of the prohibition. You are not asked to accelerate the repayment of the debt to the bank unless doing so removes the usurious interest from you. It is not permissible to take out usurious loans for building homes except out of compelling necessity. It is permissible to purchase homes from banks if they own them and sell them under the Murabaha system.