Regardless of whether settlement occurred within the normal time period, or subsequent to additional settlement, default interest will be due.
Failure to pay will result in the remaining instalments becoming immediately due.
If you do not receive any collection document, check whether the tax address is up-to-date, whether you benefit from any exemption, or if the IMI amount is less than €10 (no IMI is due in the latter two cases).
Otherwise, you should go to a local tax office or to the tax authority (DGCI) website (www.portaldasfinancas.gov.pt).
IMI can be paid at tax office collections departments, at the post office (CTT), at credit institutions with agreements with the tax authority (DGCI), via the Multibanco ATM system or via home banking.