Getting 70 versus 350 mbps.
Okay. Does seem on the low side considering that 350 mbps is only possible under absolutely ideal, not real world conditions.
However we still do not know anything about your network configurations: ISP, devices, infrastructure, and so forth.
There could be any number of reasons: hardware, firmware, drivers, cables, connectivity....
More information needed.
As a starter for you - typical setup:
ISP ----coax ---> modem ---- ethernet cable ---->[WAN port] Router [LAN port] --- Ethernet cable ----> Wired device /computer
plus (again from router) ~~~~ wireless ~~~> wireless devices.
Revise/correct the above line diagram as necessary and identify each component.
Also from your computer run and post...