I moved to a newly built apartment community of single and duplex units.

Each unit already has a CenturyLink modem/router installed that's supposed to be fiber optic, 940 mbs both upload and upload. I tested using speedtest over WiFi and was getting only 40 mbs download and 250 mbs upload.

CenturyLink sent a tech, he "cleared" the lines and I was getting about 250 mbs both upload and download. He said it's because I'm doing it over WiFi. I asked him to connect his laptop via ethernet and it also goes 250 mbs. I can't test using my laptop as I don't have an ethernet adaptor for my laptop with me.

Tech comes back a second day with instructions from his boss to put his laptop on safe mode, run the speedtest app and that's when he shows me 940 mbs. He's saying I need to have an physical ethernet connection to get the full 940 mbs. I can sort of believe that but remain skeptical why he had to put his laptop on safemode and run the speedtest app off a USB drive. I was able to get 500 mbs download over wifi on Comcast before.

In any case, he said the modem routers that CenturyLink are crap and that I should buy my own router and a better router will give me better speed over WiFi.

My question then is: Will a better router give me a better speed over WiFi? If so, which is the best router to get that will give me the best speed over WiFi? I don't mind paying some money for the router since I'm already paying for the 940 mbs service. Of course, I'd like the router to give me features like tell me who's on my network and to block devices. Also, I'd want the option to set up a guest network for visitors.

Thanks in advance.

First, test it with a GB wired connection if you can.
That will determine what is possible.
Second, your performance aka speed will be as good as your worst connection.
Third, note that the 5.0ghz band utilizing the AC protocol on both the client aka laptop AND on the "router" (actually just the wifi portion within the router) will give you the best preformance.
If you are using an old laptop (or other device) configured with the 2.4ghz protocol it may have a stronger signal but it will be slower to much slower.
So it is a matching game of Wi-Fi on both the "router" and the "client" device.
Speed can vary greatly on a number of other factors like interference, distance, and object in between the AP and the client.
Without know those other details any recommendation would be incomplete.

If the CenturyLink tech said their routers are crap, I would believe him. While I don't claim to know which equipment is "best", most quality wifi routers will let you make settings that provider-supplied equipment won't. I just went through a fresh-hell set-up with HOA-contract Spectrum. Their supplied router does not have a Guest feature nor the ability to set a channel. But it does have great reporting-to-the mothership features. I will be supplying my own when I come back here in the fall.