for starters Ethernet to your modem is a bad idea as your computer is not designed to open to the internet. please plug your WiFi router back in. unless you know how to define firewall rulesets. deny all incoming, allow outgoing, allow routed, setup a dhcp server, allow forwarded ports, etc etc etc. need i say more? if windows do a tracert to project 1999, if Linux traceroute. verify your findings with
https://www.ultratools.com/tools/traceRoute
add some helpful things like ad blocking dns or ad blocking host files to clear up your internet connection.
usually p99 uses less than 5Kbytes per second. monitor your network connection and if it's more than 50Kbytes per second you have bigger issues than your isp being poor, possibly trojans or rootkits aka malware/virus. usually from being directly connected to the internet with no protection.