[6] "aggregate turnover" means the aggregate value of all taxable supplies [excluding the value of inward supplies on which tax is payable by a person on reverse charge basis], exempt supplies, exports of goods or services or both and inter-State supplies of persons having the same Permanent Account Number, to be computed on all India basis but excludes central tax, State tax, Union territory tax, integrated tax and cess;
And As per Notification 12/2017 the Interest income is exempted supply in GST
hence as per the defination of Aggregate Turnover Exempted supply is included , so in your case he cross the threshold limit , registration is mandatory
Note : I assume the rental income is from commercial property