Subpar internet speeds

[sluggo]

I've had Spectrum's standard (100 mbps) speed for quite some time. Recently I was tempted to try their Ultra (400 mbps) for an extra $20 per month.
I had to turn in my old gateway because it was said to only have the ability for 300 mbps max. So I got their new modem and wireless router. What they heck, they no longer charge monthly fees so I went for them.
I've done several checks on speed. Every way you can think of. Wireless. Ethernet from router to Mac Mini. Router eliminated and ethernet cable from modem direct to Mini. All pretty much the same disappointing results. Anywhere from 62.8 to 123 mbps.
Now I'm not expecting a full shot 400 all the time. But I should be getting more than the 25% or less that I'm getting now. They sent a tech out today. To make the long story short, his meter shows 500+ mbps and the same speed test sites I was using show well over 300.
I was thinking maybe the Mini wasn't up to it, being a late 2012 model. But I called a local place that's handled repairs for me in the past to inquire. He says that he has a 2013 Mini and my late 2012 should be up to the task. He's very skeptical of the cable company's test equipment. An online search shows that the ethernet of the Mini is gigabit, which should more than accomplish the job.
I'm considering a Motorola modem said to be capable of handling 600 mbps. It's not much money either. Is this a worthwhile thing to try? Has anyone else had a similar experience with a speed "upgrade"?
Worse case scenario I go back to the 100 mbps plan, save my $20 a month and life goes on. I lived through the OS 7, 8 & 9 days with various dial ups.
He stated that having my own modem is the way to go, as he does this and gets the desired speeds.

[Paddy]

From my experience in both the US and Canada, the modem/router combos pushed by the ISPs are generally bottom of the barrel; they may be ok modems, but they're almost universally pretty lousy routers, since a good router tends to be more expensive and have a lot more features than these things. Is yours a combo unit? Which one? Your mileage may vary, of course. There is a list on the Charter/Spectrum website that shows all their authorized modems or modem/router combos. I'd definitely recommend separating the two for best results. There is a Motorola modem (only) on the list, which does get very good reviews: Motorola MB7621. Then decide what your router needs are - much will depend on the size and construction of house etc. We have an Asus RT-AC5300, which works very well in our house (over 2 floors and basement, new wood construction). Other options are things like the Google Wifi etc.

However, before you do any of that, try to test with some different devices. Do you have an iPhone? Download https://apps.apple.com/us/app/speedtest-by-ookla/id300704847 and run a test with that. That will give you a good idea of your wifi speeds (make sure the phone is ON your wifi network, of course!), and then perhaps get someone (friend, neighbour) over with a couple of other devices - Macs, PCs...to see if they get better results than you do. If not, and particularly if you see lousy results with a Windows laptop running Win 10 or another Mac running Mojave or even Catalina, then you know it's not your Mini that's the problem - and Spectrum's test equipment is dodgy too! Of course, if they get significantly better results, then it may not help to upgrade the network equipment - there may be something that needs tweaking on the Mac Mini.

BTW - Charter/Spectrum have a history of providing lousy equipment: https://arstechnica.com/tech-policy/2018/12/charter-users-who-didnt-get-promised-speeds-will-get-75-or-150-refunds/

[sluggo]

Thanks Paddy. I actually went ahead and ordered the Motorola MB7621 a day ahead of your reply. For $77 at Amazon I thought I'd give it a shot. Easy to return if the experiment fails. It's up and running now. Test speeds are up and web page responsiveness in the browser (Vivaldi) seems much quicker.
I was using an Arris modem and a Sagemcom wireless router, both from Spectrum. I'm still using the Sagemcom but am considering going back to the TP Link C9 Archer that I own and have on hand. But the wireless aspect seems to be working OK at this time and I don't feel like changing passwords again on the Nest thermostat, iPad, iPhone, wife's Alexa, etc., etc. So I'll ride it out for a bit.
As far as the test speeds on their meter, I'm told by a different tech than I consulted the first time that they usually do show significantly higher speeds because they're not running an OS or any major drivers on it.