Q.What is wrong with my Devils Ivy??
I bought this plant a couple of months ago – and when I bought it, it had fungus gnats (all gone now), but the plants leaves keep turning yellow then brown. I know I haven’t over watered it but I’m not sure what’s the problem.

From the photo the plant appears to be underwatered, as indicated by the wilting and some dried leaf tips and leaf edges. The yellowing can also be related to the water deficit, and possible nutrient deficiency.
A soil moisture meter can help with taking the guess work out of when to water and how much.
You may be under-watering in order to avoid water spilling out of what appears to be a small tight fitting saucer under the hanging plant.
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-moisture-in-plants.htm
https://www.domyown.com/luster-leaf-rapitest-mini-moisture-tester-1810-p-7732.html
https://www.domyown.com/dr-earth-organic-all-purpose-fertilizer-p-9485.html