What do you do if your team just doesn't split push?

split push yourself ( duh )

pick someone with hard engage like sejuani/amumu/naut/sivir etc etc, force the fight mid lane, you'll win because everyone is there and you brought more aoe, and then just push down mid and take something in return.

you just cant get your team to do the right thing, even with pings and commands, they just dont listen, you flat out, have to carry them.

Tell them to upgrade their god forsaken trinkets, place down wards wherever they can, and you attempt a split push yourself. It's very dependant on where those wards are because you might just die if you don't see an attack coming but it pays off. Also they'll probably send about 2 people after you which should give your team an advantage against the remaining 3.