Surface Pro 3 - Windows 10 - WiFi (slow or limited connectivity )

Well it's just a theory;-)

But AC do run on 5G, you may need to disable AC (using N on 5G instead) to prove it's AC and not 5G.  And even if N on 5G works, there is no guarantee that AC on 5G would also work.

The quickest way to ensure this is not the problem, though, is to check what channels are available on 5G on your AP.  You should be able to login to your AP and change the setting. If you are seeing proper US channels as listed in the link below -

https://en.wikipedia.org/wiki/List\_of\_WLAN\_channels

Then your AP is most likely up to date. From there you can try to use a fixed channel (instead of Auto) and see if things improve.  It's recommended that you use different SSID for 2.4G and 5G for easy troubleshooting.

Not sure which APs you bought, but I won't be surprised if the firmwares are not updated enough to cooper with latest changes.  The reason your other workaround works can just be that both the your USB stick and AP are on the same (old) implementation (which ironically works). If you have no preference, I would recommend later models from Asus and Netgear, they do update firmware frequently.

BTW you should be able to run N on 2.4G, in practice you probably don't lose too much speed.

Additionally I have the ask the obvious - did you get your Surface locally - Is that an US SKU?  It's already a known issue (another great pain) that Surface will (now?) stick to the Wifi Channel where the SKU is supposed to be sold.  As far as I can tell, this issue has not been fixed.

I don't work for Microsoft, just try to help;-)

I like your theory but I think I can disprove it.  The problem for me isn't 5G .  The problem is AC.  I had a Dual Band 2.4 and 5G wireless N router and everything worked fine.  2 weeks ago I upgraded my router to a triband wireless AC router and since then it's been pure wireless hell since.  So, just to make sure it wasn't an issue with the router,   I bought a 2nd router, a Dual Band AC router from a different manufacturer and the results were no different.  The 2 routers were even purchased in different countries; I bought one in Canada and one in the USA and they were both awful with thee SP3 - like to the point of pretty much non functional.  Yes I could connect to the internet (intermittently) but connecting to other devices on my network was disastrous.   So, I tried two more things with both routers:

1. I manually set the internal Marvell Network adapter to operate at 2.4G only and the problem disappeared but obviously I didn't get any of the speed advantages of AC or Even N.
2. Next I completely disabled the internal Marvell adapter and plugged in a USB AC1200 network adapter and the Surface performed flawlessly with both routers on the AC 5G band and I got true 822mbps connection speeds.   I have 2 SP3 devices in the house and had exactly the same results with both.

The ONLY conclusion I can reach is that the Marvell Card used by Microsoft is an absolute piece of s. ........ when it comes to compatibility with the AC Wireless standard for which it was built.  Ands I must admit I'm really not happy about it.  This is supposed to be a flagship device and it is not performing like one.  That I have to stick a dongle out of the one USB port to make it work is ridiculous.  I upgraded from a SP2 after a year and looks like I need to swap out again, Only this time I WILL NOT BUY A SP4 - I'll be going to the Lenovo Miix 700  - Shame on you Microsoft!@