How to reach Umbral Plains without ports
As you may already know, getting a portal to Dawnshroud Peaks will greatly reduce your travel time to Umbral Plains. Without a port to Dawnshroud it's a rather lengthy run to make it here! The best place for you to start this long journey is in The Nexus.
Shown in my map above you'll want to travel north through The Nexus to reach Netherbian Lair. Once here you'll want to travel directly south through the zone until you reach the Dawnshroud Peaks zone line - shown in the map of the area I have below.
Once you're in Dawnshroud Peaks invis up if you have it and run southeast to the Maiden's Eye zone line. The route is marked out on my map of Dawnshroud Peaks below if you're having trouble with this part.
After arriving in Maiden's Eye - things will start to get a lot more risky for you. There are some enemies in this zone that see through invis and they can cast rather deadly debuffs on you while you're running away from them such as Root or Snare. I highly recommend you avoid every enemy you can and run as fast as possible to the Umbral Plains zone line.
I recommend you follow the blue line on the map below rather closely while you're in Maidens Eye. This is generally the path where you'll encounter the least amount of enemies, thus making it the safest route you can follow.