The license plates must be fixed on with the respective screws or metal fixings - garages, chain service places, etc will typically do it for free or a few euros, if you use the metal fixings. You may not pass an IPO otherwise. You will be expected to change them if damaged or faded in due course, which happens easily with the plastic ones.
As @crazy-russian says, you must display the insurance and the IPO dísticos (little squares - they are different squares, and the green one is now white as of this year!) and carry the rest of both the documents with you in the car, at any time on the public roads. If you are stopped, and don't have all these, you will have problems - and Police in towns may check parked cars, too. If you have old ones that there is a good COVID reason you couldn't renew, then that may just be OK - but IPO centres and insurance brokers have never closed, so they may not be kind.
These should be displayed properly, in the little plastic pockets (which insurance offices, IPO places and car associations, etc, can give you for free) not just taped on etc. They need to be clearly visible to someone standing at the side of the car, not in any way obscured or hard to access.
I have used Ged at https://gedinsurance.com/ for all our insurances - he only works with Fidelidade, but comparison sites showed that to be cheapest for all my stuff, so I found him.
(Comparison sites do exist, in Portuguese of course. You could try, though I have not used them to recommend them:
A broker should be able to email or fax a document which confirms your insurance is paid and you're waiting for the dístico - but we haven't noticed post problems since last year, in our area, at least...
Another Portugal-specific thing: Your reflective vest must be kept in the main body of the car, within reach of the driver without her/him having to get out to get it, so for example in the glove compartment.
If I sound picky, it's because I was stopped some years back, in a rental car which was not compliant (before I knew to check this) - it was missing a dístico, had none of the full documents, and the gilet was in the boot. I was kept for hours in the baking sun in the middle of nowhere with no phone signal, trying to explain this, and the rental company was no help afterwards either. So best to make sure you keep everything in order!
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