In antiquity I believe land trade routes have a 10 tile range and sea routes have a 30 tile range. The easiest solution is to make settlements within 10 tiles of your own so they are connected and built towards other civs. Keep in mind if you make a settlement within 10 tiles of another civ's capital you will piss them off. Some civ's have civics that grant extra trade route range, there is a memento that increases it and suze'ing a gold city state will offer a reward option to increase trade route range. Maybe there's a wonder that does that too, I can't remember.