Conventional reality (sammuti sacca/ pannatti) is just a concept and not real.
Non-real dhammas don't exist and therefore cannot be anicca or dukkha.
According to Abhdhamma:
dhamma = paramata + pannatti
paramata = sankhata + asankhata
sankhata = citta + cetasika + rupa
asnkhata = nibbana
dhamma = citta + cetasika + rupa + nibbana + pannatti
All the dhammas are anatta.
Anatta means devoid of essence/self/ownership/hegemony.
Only the sankhata (compounded) is anicca-dukkha.
Nibbana and Pannatti are anatta only.
Nibbana is real but not compounded. Therefore not anicca-dukkha.
Pannatti is non-real. Therefore not anicca-dukkha.