As of update 48.1 we cannot use the Tardis to travel TO nor IN a modded dimension such as Twilight Forest, this was, if i remember correctly, possible in 1.7.10 where there was no coordinate travel.
If there is a way I can implement a custom dimension/planet and is not too complicated, then i'd be happy to hear it.
Step 1: Go to the modded dimension you need to go
Step 2: Summon your Tardis there
Step 3: Go in to your Tardis
Step 4: Go to the monitor and add a new waypoint
Step 5: Now you can go to that dimension when ever you want
Step 6: Repeat with other modded dimensions you have
That's the first thing I did when I got to a modded dimension. The problem is we can't travel INSIDE the dimension, like if we use Journey map to find coordinates and then type them into the monitor, it won't "find" the coordinates as it's not familiar with the dimension for some reason. The part where we can't travel TO the modded dimension is referring to that we can't find it in the planet selection.