This problem appeared out of nowhere two weeks ago for me, but first, there wasnt even a notification about ensuring ports. I was just unable to invite any player, or be invited by anyone, and starting missions got really slow, because game tries to find a party for like 15 seconds before starting a mission anyway, when Public is selected. But even though it cannot find any party, people still seem to join already started missions, which seems to be really strange.
I started getting this "Please ensure..." notification yesterday and spent about 8 hours trying to fix the problem, but with no success. I think, I tried everything: creating exceptions, checking if my ports are open, disabling/tweaking stuff in firewall, changing ports in Warframe settings and many other things, but none of those seem to work for me. The only thing I haven't tried yet is using VPN, because I don't want my ping to pierce the heavens, and I don't even know if it is allowed, or I can be banned for doing it.
I have a rather silly theory which actually may be true and an actual cause for the problem which I and many other players are having.
So, long story short, I live in Russian Federation, and there is a thing called The Federal Service for Supervision of Communications, Information Technology and Mass Media, which we simply call RKN. Last year because reasons apparently it tried to block acces to a messenger "Telegram" by randomly banning hell of a lot ip adresses. I think, some days it was like 1,5 million ip adresses banned. So, I've done a bit of research and found out that arter this RKN bombardment many Russian Warframe players started to get this problem. It's not like they weren't getting it before, but before this problem wasn't so widespread. So, the thing is, RKN is still out there, doing it's stupid blocking thing, and this is why many Russian players still get this rather inconvinient problem.
Sorry for babbling, I'm just getting annoyed that it appears there's no salvation except maybe sacrificing smooth gameplay for an ability to actually play with people