Okay I know exactly the issue. Yesterday I was playing against the most roll happy defensive player ever and he knewv what he was doing. He used sheik, zss, MegaMan, the works.
As a Marth player here's what I did for relative success (winning more than losing).
1- Let's break this fantasy right now: you are not going to be applying rush down pressure in the neutral game and win. Nair whiffs on a lot of air to ground situations, fair is just too laggy to use on a standing opponent... overall in neutral you don't want to throw too many things out there. You can advance, shield canceling dashes and dropping shield into dilt or filt, but in general you don't want to risk rushing with a move cause, well, you're gunna end up hit. Instead, you got to get your opponent in a position that favors you. Whiff moves from a distance, encourage them to press buttons outside of their range. This is where you strike.
2- when you land a hit for the sake of everything this is where you enter your offensive. Pretty much all of Marth's moves pop the opponent back into the air, and a lot of them can beat opponents moves due to their range and speed. You gotta be careful not to let an opponent break out for free. Make them fear having to fall back on stage. RAR bair to fair works well from full hop, fair itself is great from full hop since you don't really have to worry about landing lag, his dash attack pops them right back into the air, and more. It's a fun position to be in.
3- rolls. This is one of the most difficult things to deal with as Marth. Rolls are so quick that you can't even get out a reversed f smash on time to hit them before they roll a second time after they roll through you. It's here that you need to use pivot tilts, dilts, grabs, and dancing blade so as to catch them quick enough and set up to pop them back into the air. Eventually you'll see their tendencies after a roll and be able to punish accordingly.
Lastly... It's online. Lag exists. I fought a Marth v Samus and could only hop over projectiles and watch them roll away without being able to do anything. Marth works with precise inputs, that don't really exist online. You just kind of have to deal with it and start inputting things way early. It's typically all you can do with lag.