You may have just found a plot hole!
In Doctor Strange (2016), Mordo clearly states that the ring is used to travel throughout the multiverse.
Mastery of the sling ring is essential to the mystic arts. They allow
us to travel throughout the multiverse.
There is a but.
Mordo continues as:
All you need to do is focus. Visualize. See the destination in your
mind. Look beyond the world in front of you. Imagine every detail. The
clearer the picture, the quicker, and easier, the gateway will come.
We know that Strange knows Earth and as an Earthling he can visualize places on Earth. But during that scene where he asks Tony Stark if he can control the ship, he knows they are going somewhere which is not The Earth.
They were going to Titan. That’s why he is not able to open a portal. He doesn’t know anything about that place.
After arriving, he used this ring-power of his to aid Spider-Man and Mantis on multiple occasions while they are fighting Thanos.
Which brings us to the final part of this answer. It is either related with future plot line which the directors will point out in Avengers 4, or there is literally a plot hole because Strange was supposed to be able to open portals throughout the multiverse.