Some devices do not enable use of the Spotify standard app equalizer. For example the LG G2 & G3 models.
There are many 3rd party equalizer apps in the Play Store, but I have struggled to find one that works (and does not introduce bugs, malfunctions etc).
Finally I have one : Equalizer Fx
It is free, works perfectly, many presets to select from.
It is still only a 5-bar equalizer, but the presets are sufficient.
You can get the Pro version for 1.19 euros to remove ads if you use your device online a lot (I haven't found the ads to be intrusive in any way).