So I'm going to try to break this down best i can.
first lets forget about the "70mbit internet" it has literally nothing to do with this.
Your issue is simple, your wifi router EA6350 simply can't push that much data over wifi.
Wifi has 2 bands 2.4/5.8
your camera's only support 2.4 which is bad, 2.4 is the slower band with limited bandwidth and limited channels. buying a faster "dual band" router will do nothing. the camera's will not use the faster 5.8 if im reading the limited docs on the specs for the camera correctly.
Wifi on 2.4 is a half duplex medium,which means it can only talk to one device at a time. so to break it down, if its talking to one camera, it can't talk to the next one, till the first one is done talking, (high level here). Also depending on how good the connection is between the 2, contributes to how fast they talk to each other and before the router/AP can talk to another camera or your laptop (they are all waiting in line to talk). Since your using a NVR, the camera's literally never stop talking over the wifi since the NVR even if its using motion detection still needs a live stream to detect motion. hopefully that was clear.
wireless camera's suck, yea they make install easy, but wifi is a prick and is a shared spaced so alot of it can be interfered with and you can't control that.
options,
if you can hardwire any of it, do it, if you can't run a network cable, look into Powerline adapters, or MOCA adapters.
your other option is if you have the 2.4ghz channels available (remember anyone can use them)
is to buy another AP/AP's (access point, wireless router), set it to a completely different SSID (wireless name) and on a different 2.4 40MHZ channel (can only use 1,6,11 without them interfering with each other) and move all the camera's/dvr's to use only that SSID /router.
since your trying to use extenders (horrible idea they have to rebroadcast on the same channel unless you bought the dual band units, which you can use to connect to the main router on 5.8, while providing a 2.4 signal to the camera's (might be an option)
there is alot of testing here unfortunately. moving a wifi ap/router 3 feet can make a huge difference in signal levels.
there is a reason i spent 2 weeks hardwiring my entire house.
a nvr setup with 8 camera on wireless is a disaster waiting to happen. as you have found out. it can be done, but most consumer gear is not up to the task.... good luck.
if you want to give a certain AP setup a try, get the 3 device Netgear orbi,( no idea on the size of your house) and see if it has enough power for you, this may get it done,as it backhauls the data over the AP's on 5.8 while giving out a 2.4 (like i mentioned above, this may be your best shot and easiest, may not be the cheapest.
link to orbiand again your internet only comes into play when your trying to remote view the camera's. everything between the nvr and the camera's is local and never leaves the house thus never using your 70mbit download speed.
to add another wrinkle 70mbit is your down speed, when your trying to view your camera's remotely thats actually a upload, which is generally much slower something like 5-6mbit. you will never be able to view all camera's at high resolution remotely.