That's a unique combination of factors that has not been asked about before
. It's also a good example where you can read official government websites all you want and you are none the wiser.
In this case, they either want to see a
RT-PCR test (or similar NAAT test); or a rapid antigen test; or a valid EU Digital COVID Certificate; or a valid vaccination or recovery certificate issued by a third country, under reciprocal conditions [as of 17 September 2021].
Since you don't have an EU DCC and if you don't take a test as required you would have to have a "
certificate issued by a third country, under reciprocal conditions". What does "
under reciprocal conditions" even mean in practical terms??? Do the UK's NHS certificates qualify (perhaps)? The US CDC cards (unlikely)? Or does it refer to the 16 or so non-EU countries whose certificates are officially recognised by the European Commission and hence by Portugal, with reciprocity, and with neither the UK nor the USA among these countries (most likely meaning of this line)?
When is your flight due? I'd try to get reliable information from the airline staff at Frankfurt, or perhaps get a rapid antigen test for "just in case".