@Big_boss yup i did check it… I was setting up my Plex server (or rather FW for it), so it was very easy for me to test if it’s working or not. (playing music/videos on my phone via lte connection) .
@Vojtech_Pihrt seems to be working fine ever since. (I just now removed all my rules and recreated them via /etc/config/firewall + /etc/init.d/firewall reload, again successfully).
Maybe something to do with configuration via Luci… that one was rather weird for me. It said i have uncommited changes even though everything was saved/applied and verified via iptables directly.
Once I was able to get rid of that by deleting the rule via Luci and even “restoring the rule” (or whatever the option is in that “uncommited changes window”), and then 2 or free firewall restarts, it started to work as expected.
I understand this is not very helpful story
and there is a real (and probably high) possibility the problem was with the moron behind the keyboard (that would be me if there is any doubt) 
Anyway everything seems to be in order right now, whatever the issue was.