The internet is extremely slow over wireless and/or cable

Our internet has become really slow in the last few days (0.73Mbps tops).
It was never very good, but it has become so poor lately that we got our telco (Telstra) to send out a technician to test our line. Before he came we took our modem to a friend's house and checked that it worked with a wireless dvice there, and it was fine. The technician came out and he connected his computer directly to the internet and he got 3+Mbs so it was not the line that was at fault. We found while he was there, that if we did not have the wifi turned on, our computers worked fine. He said there was a problem with a one of the mobile devices that must have a virus, or a program running in the background using the bandwidth.
When he left, we tried to turn on the wifi on the modem, but even with with all our devices turned off the internet slows down including those computers on cable to the above 0.73Mbs. It seems just turning the modem's wifi on slows the flow of data.
Because of this we thought this must be a modem problem, so we bought a new one. This one behaves exactly the same or even worse than the old one, as now not even the cabled computers work quickly even with the wifi switched off.
So far we have spent $130 on a telco technician and $200 on a new good modem and still the problem persists. As we are a business that runs on the internet, this is a disaster for us. We don't know who else to get in to fix it or if there is anything we can do to fix it ourselves. Any ideas please?
 
Let's draw out the connections between the devices (you cut/edit to make this correct)

ISP==the modem --- a router --- some pcs -- (any daisy chained to others??) --- pcs

Add a wifi router if you have a second router.
 
ISP==Netgear modem router-desktop computer

That's it. We tested the speed with this configuration and the speed was around 0.7 Mbps.

When we tried the same configuration using the old Netgear modem router, the speed was 2.5 Mbps, but only while wireless was switched off. With wireless on, the speed dropped to next to nothing, and in fact frequently dropped to 0.
 
NB when the wireless on the old router was turned on we had no devices connected as all were powered off. Why would turning the wifi on with no devices connected cause the speed to drop from 3 Mbps to almost nothing.
It was worse with the new router, with slow speeds on cable and wifi.
 
Disable WiFi for now and use a wired connection.

then download, install inSSIDer

You can then enable WiFi, run the program & click the network Tab.
Look for your network name and see how many others there are on your channel.

If you double-click on the column heading Strength, the strongest station will be at the top, and it really ought to be yours. If you know how, get a screenshot and paste that into your follow-up
 
Other settings to improve networking:
  • tcp1323 - - sets network scaling
  • tcpSACKopts - - reduces tcp overhead
  • Set MTU - - stops packet fragmentation
save all to disk. For both TCP** files, rename and delete the .TXT suffix.
Then double click one at a time and allow the changes to be made.

For the Set MTU, search -> runas /user:administrator cmd
and provide the password
then PASTE each line from SetMTU into the b/w window and hit enter.
 

Attachments

  • Tcp1323_Regedit_file.REG.TXT
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  • TcpSACKopts_Regedit_file.REG.TXT
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  • Set MTU.txt
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