Wifi Speed Very Slow After Modem Replacement?

I have optimum online internet. The modem was an arris 802g modem that was given to us by optimum a long time ago when we had internet installed. They also lent us a router... netgear n600 and it has worked great. Internet has been very fast wired or wifi on 2 floors.


But they started charging a monthly fee for modem so i went and bought a modem. The netgear cm600. I bought this because i read its very good and more expensive than the cm500. I had it installed and set up with optimum by activating it and it worked good when i set it up.


However, few days later, the internet in the 1st floor is extremely slow. The modem and router are in the 2nd floor where it always has been. So we asked the people on the 2nd floor who we rented to if they were having internet issues and they also said yes. Then they i believe unplugged the coax cable and maybe powered off the modem or router and plugged it back in. Then the wifi on the 1st floor works fast again. However, not that long later, the internet is very slow. I checked on speedtest and even though we got 30mbps when checking on laptop.. the upload speed is less than 1. Right now as im using laptop on wifi, its extremely slow. I cant even upload anything because of this. After we did the installion of the new modem not long ago, i checked speedtest and it was like 58 mbps down and 25 mpbs up which is very good.


What are we suppose to do here? Is this a router issue or a modem issue? The thing is the modem and router are on the 2nd floor so we have to ask them if we can check the modem/router etc. Last time we ask them and they checked it and the issue got fixed... probably by them adjusting the cable.


What are our options now? Could i reset the modem or router though 192.168.100.1? When i go there, i entered admin and password and it seems to be able to log into the netgear router options. Also seems like i could reboot or reset it without asking them to check the modem or router?


The thing is before we replaced the arris modem with the netgear modem, i was told there was rarely if any issue with wifi on the 1st or 2nd floor before this. So is this a modem issue or router issue? However, how can it be router since well nothing is changed with it right? They are put where its normally put.