With a cable modem look for a high frequency bandwidth splitter
I use a 2500mhz (2.5 ghz) splitter I got from Home Depot
but as MrYoink correctly states
having too many splitters and couplers is detrimental to the cable modem/phone line
signal
cable tv needs less DBs of signal compared to a cable/phone line modem
Try entering 192.168.100.1 in your Internet address bar
This should take you to the modems internal menu/stats screen
look for status screen
it will show you the Signal To Noise ration measured in decibels
On mine it gets about 35-36 dbs
which I find to be borderline
My cable install suffers from too many splits and couplers itself so
I have been dealing with this issue myself
try to use high frequency splitters
use the better quality cabling where you can and try to keep the runs short
if it is possible to run one two splitter where the cable comes in then
one leg to the cable modem and one to the rest of the TVs
make sure all connections running to the cable modem are tight
use pliers if need be to tighten
BTW
I was up to 5 a.m. dealing with just this problem
I managed to get my service back online but I am monitoring it by checking cable modem internal software
I might have to run a new line which of course I am trying to avoid since I am lazy