I need to connect more than 80 mobile devices to a normal, ISP-provided router. Not all are being connected. Do I simply need an expensive router? Or are there configurations that need to be done?

I don't know if this is the right sub, if not then I'd like to apologize and would be very thankful if someone recommended a more appropriate sub. More context:

• Speed is 75 mbps (yeah, third world country. it's considerably very fast already)
• We've done no configurations to the router. We can access it via Ethernet cable with a laptop.
• The router is really basic. I'm not really able to provide the exact model anytime this week (it's not mine), but think of it as the most normal, non specialized, and inexpensive router.