Reset modem/router

Comcast, my cable provider has been upgrading download/upload speeds recently and I am no longer able to receive these higher speeds via wireless. We are receiving 100mbps from Comcast but only getting about 20-25mbps through our mobile devices. When I connect our laptop with Ethernet cable the speed increases to about 75mbps. Comcast tells me they cannot fix my problem as it is not their router, and my warranty with net gear has expired and I don't want to have to pay for resolving this issue. Can someone tell me what I need to do to change the settings on the modem/router to receive closer to the 100mbps speed I receive from Comcast. The Netgear cable modem router is a Model 6300 AC1750.

Thanks,

Rick