First check that the ESC is in Lo voltage cutoff mode. The instructions can be found here: https://www.tamiyausa.com/media/file...-1207-ad8e.pdf

When the ESC is turned on, you should get a short beep. Lipo mode is a long beep. But if you don't know the difference between short and long, you won't be able to tell until you toggle between them.

If that doesn't work...You didn't mention what charger you're using and what settings you were using to charge the battery. I've never driven that Tamiya model before, but based on similar Tamiya brushed motor models, you should probably get ~20 minutes.