Data Drops when tethering
It could be that your ISP has different APN configs for normal data usage, and the other specifically for tethering plan. As such I'd recommend fistly to check the APNs and see if there have the DHCP setting enabled, sometimes the  device is not getting the IP address assigned automatically hence drops in connection whilst trying to connect.

If its the case of the APN for tethering then also check the related setting and also be sure you have an active tethering plan.You could also use adb to prevent the device from revealing the tethering status i.e modifying settings get global tether_dun_required code. See more here in a related post.