Increasing Gigabit speeds

I’m having similar issues with my wifi, but the issue I have is with the “total of all devices being connected = the speed you should be getting” theory (like you said 3 devices using 300mbps = the 900 mbps expected speed).  I had a tech out all day yesterday trying to fix my Wi-Fi service.  I’ll skip the many issues he encountered (including less than 50mbps when hooked up directly to the white box where the finer comes in to the house) and nothing added up to the multi device-total speed formula.  I recently upgraded to the 750 from, 300 and there was absolutely no increase in speed, which there should be if my devices that were sharing a 300 straw and jumped up to a 750 straw.  While trying to sort it out, the tech here was on the phone with whoever is at their office and they moved the whole thing up to 1gb, with absolutely no change in speed to any device connected to the wifi.  The laptop hooked directly to the modem was the only device getting above 500mbps, but no where near 1gb.  They were making changes and adjustments and while doing so (and after setting it back to 750mbps) the wifi speeds shot up to 400-500mbps.  As soon as the tech at my home got off the phone, the speed throttled back to 250-300mbps.  The tech could not explain this.  Even with only one device on the wifi (because they changed the modem and nothing else had been connected to the wifi yet) and holding the device 1 foot from the modem, the speeds would not top 300mbps. Again, the tech was at a total loss to explain this and tried with his laptop on wifi and his phone as well with the same results.  On the phone today, I was called a liar for saying the speeds went up while the techs were actually working on it, then slowed way down when they were “done”.  I was repeatedly told that they don’t guarantee any specific speeds on wifi so I asked why they would sell me an increased speed, knowing full well there would be no change in the actual speeds I was seeing on my devices... of course they had no answer other than to tell me they don’t guarantee speeds on wifi.  Unfortunately I’m stuck in their contract until the summer and, even though I’m going on 5 years with them and a crazy-long list of issues over the years, they refused to let me out of the contract early and want more than $200 to break it.