Hi, I have two internet service providers (DSL, LTE) I would like all downloads to use the DSL connection and all uploads to use the LTE connection, is this possible?
and where do I start?

That is possible at BGP... your home internet providers not support that at 99% at your service plan.
It's material from MTCINE course.

For general ISP you can try few policy of doing LoadBalance, check the presentation of Tomas Kirnak at point 2 this post viewtopic.php?f=13&t=166412&p=818011#p818011 .
TL;DR nth, PBR, PCC, BandwidthMonitor - this ways.