You can use wifi, but the OTG connection is much more reliable. One thing to remember though, with the OTG you have to turn off the wifi connection, boot the app, remote, drone, and then OTG - in that order - for it to work.
Can only recommend you to have a look at DJI Spark Accessories & Tips - Spark Help
If you don't have a high end android phone, OTG cable can help you but it is not supported from DJI.
At first I would try to use wifi first and if your are not happy, then go for OTG cable.