MacBook Pro M4 Max – Wi-Fi unstable and unusable. Help!

There are no pervasive changes to Wi-Fi due to MacOS 26 Tahoe.

Nothing else may have changed in the part of the network YOU control. But most Wi-Fi network have neighbors that share the same airwaves, and their changes may change your operation.

Wi-Fi Radio Signals from your Router fall off as the cube of the distance. Provided you are close to your Router and have clear line of sight and no competition from neighbors' Routers or known interference producers like microwave ovens, you should not be having issues.

All others need to do a more careful analysis to find out what is happening.

Wireless diagnostics:

Hold down the Option key while you click on the Wi-Fi icon on the menubar to open up the tools for investigating and fixing Wi-Fi issues

Choose "Open Wireless diagnostics", which opens the wireless diagnostics Assistant, but does not proceed.

NB> Wireless Diagnostics is an App that puts up its own MenuBar.

Using its Window menu, there are about eight different things you can do from here, but the top-level is to choose Diagnostics off the Window menu, or simply click (Continue) to do a quick check for Gross misconfiguration or operating problems. Your Admin password will be required. If any recommendations are shown, you should consider then seriously.

Next is to hold down the Option key and click the Wi-Fi icon as before. The screen that opens shows operating parameters of your Wi-Fi network. There is a great deal, of valuable information in those numbers.

Readers are eager to interpret what is going on from the values there. Screenshot, transcribe, or photograph the results and post back in a reply on the forums. Looks like this older one.

You have good signal strength (RSSI) of -54 dB, where around -40 is typical right next to your Router, and -75 is unusably low.

Noise is respectably low at -97, suggesting no interference from neighboring devices.

You have connected to the Router-proffered 40 MHz connection on channel 44 in the 5 GHz band, which encompasses everything from number 44 through 48.

Your Router is using 802.11ac rules, and you have made a connection at 256 patterns per signaling interval, the highest available with both your Mac's antennas at once. (MCS index 9 and NSS 2). You are obtaining transmit rate of 400 M bits/sec, the highest available with those parameters using an 802.11ac Router. (A faster 802.11ax router would boost that to around 458, assuming all else in unchanged.)

There do not appear to be any Hardware issues shown.

There does not appear to be massive interference from neighboring Routers.

You could Option-Open Wireless diagnostics again and choosing the Monitor function, or choose the Performance window, which will show real-time performance results:

Alternatively, consider software issues.

Hi. Just a quick guess: did you try disabling things like 'private wifi address', 'limit ip address tracking' and the like?

Poisoning Internet performance:

By far the easiest way to cause poor performance, instability, overheating and crashing is to install ANY third-party speeder-uppers, Cleaners, Optimizers, Virus scanners, Bit Torrent, or a VPN that you installed yourself.

¿Anything like that installed?

a self-assigned IP address is a pseudo-random IP address the Mac gives itself when it starts up. this allows it to be uniquely identified on the network, but is not a Routable address suitable for connecting to the Internet.

Your Router immediately responds with a "good" local IP address that is from a range of DHCP "private" IP addresses, which can be used by your Router, acting as you agent, to get stuff from the inter for you. (e.g., likely half the computers in the world use a good, strictly-local IP address in the range of 192.168.xxx.yyy)

When you see self-assigned IP address immediately after startup, that means "No Router will talk to me and give me a good local IP address to use."

Punchline:

Transitioning from a good local IP address to a self assigned IP address should not occur within minutes in normal operation. (It might occur after your DHCP lease expires, but that should be longer than a workday, and should renew automatically.)

Try the Performance window, and see if the graph provides any additional clues.

Ttry running the Mac diagnostics:

Use Apple Diagnostics to test your Mac - Apple Support