not reviving gigablast speeds

so to start from the beginning we had just found out that we are able to receive gigablast speeds we where originally paying for 300/30 which we wanted more we called them that day and called to see if we how much it is they said it wasn't much for us to upgrade but that we had to upgrade our modem or they said that we can rent theirs we didn't want to so we went to buy our own modem we found the Netgear cm1100 and i also allready had the nighthawk ac1900 and did a bit of research and I pulled the trigger when it finally came and we called to set it up and upgrade our service we set up every thing and plug the first PC directly into the modem and we got the speeds that we should be getting and I hung up after I got all of the other devices on the network the other 3 pcs that where not able to devise the gigablast speeds now to answer some questions, yes the pcs can receive gigabit speeds and that the only PC that can is actually the slowest PC in the house, so i used the one PC that can receive the gigablast speeds to check all my runs in the house and everything checked out. so I tried to delete drivers' devices and i was out of ideas so i called cox and pf course they said everything looks good on their end and they even sent a tech out to check the tap and he said that he was able to pull 1.5Gbps. so after that, i was just still out of ideas and thought maybe it was something with the modem and or router I called Netgear and they ran me through a bunch of tests and they said everything is fine. so i ended up calling again and they transferred me to a Lvl. 2 tech guy and he pretty much made me do the same stuff the basic people had me do he even had me do some advance stuff wich if i wasn't that tech-savvy like my mom he wouldn't have been much help, so he ran me through all these same things and he couldn't find anything wrong and that he was completely stumped. then one day my brother was messing around with his PC and broke something and we thought the only logical solution was to wipe is windows i told him that was very dumb but he said whatever but strangely after that he ran a speed test and was able to get gigablast speeds. so i called another tech and they gave me a Lvl 2 guy and he was the first one that wanted to remote into my PC he also just did a bunch of stuff i did and was super helpful he literary tried everything and wasn't able to do anything he even told me he was completely stumped as well. so after that, I'm completely clueless and i don't have a clue on what to do the first Lvl 2 tech told me to buy a pcie 2.5 gigabit ethernet card and i have a feeling that will work but i don't want a fix that requires me to spend more money so far our only fix has been to completely reinstall windows which is not a reasonable fix so im hoping someone knows something i haven't tried and is the solution thanks!