I need help figuring out how to add a timer to my mobile speedruns. Please someone tell me or give me some advice on how to do it.

You can edit your Speedrun video and put a timer in there. If not, then use livesplit.

Take a look on the "Timers" section in the speedrun.com resources page: https://www.speedrun.com/knowledgebase/resources

It appears that FramePerfect is the timer for iOS or Android devices: https://futuretrostudios.com/frameperfect/

As far as I know, Livesplit only works on Windows.