Inactivity deranking system.

Hi. I just got diamond 5 on solo duo about 2 months ago and kept playing a match of ranked once every 10-15 days to mentain it since i know the inactivity limit is 28 days but i was wondering what if lets say you got diamond 5 on solo duo and you are playing some ranked lfex 5 v 5 and 3 v 3 on the side, will that consider as "activity" for your rank so you dont get demoted or will you need to play the game mode that you achived the specific rank in to mentain it further. Beacicly i want to know if by playing ranked 3 v 3 or 5 v 5 flex stops my solo duo rank from getting down. Curious about normal games too but i guess those wont count towards any competitive parts of the game. Feel free to add more details about the deranking based on activity system as in the differences between ranks ( plat, daimond. master... ) and the ammount of lp you lose etc...
Thanks and good luck in your games summoners!!