MacBook will only connect to 2.4G WiFi, not 5G

You're probably running into a newer router feature called Band Steering. When it's on, the router automatically selects which band to use per device. The weaker the connection (as in being further away from the router), the more likely it is to make that device use 2.4 GHz.

Go into your router's settings. Find the correct Wi-Fi setting option and turn Band Steering off.

Next, give your router's broadcast names something easy to distinguish. Such as below (Ducky doesn't exist. I changed my actual wireless names for this example). If the current name is something like Quantum4431, give each band the same name, but append -5G to the 5GHz band. Make the Wi-Fi password for both the same as the password you're using now so you only have to remember one.

Save the changes, and you'll likely have to restart the router. Afterwards, you'll be able to choose the 5GHz band with the MacBook Pro since you'll have a choice of both Quantum4431-5G and Quantum4431 in the list of found wireless signals.

Where is the MacBook in relation to your base station?

While the 5GHz band offers higher throughput than 2.4GHz, it has a shorter range and is more affected by walls and other obstructions.

It may be that you're at the limit of the 5GHz range and that the 2.4GHz signal is stronger, so the MacBook prefers that.

it may be that the position of your MacBook on your desk is somehow blocking the signal (e.g. there's a monitor or printer between the MacBook's antenna and the base station that doesn't get in the way of the phone).

It may also be there are other 5GHz networks in range that are using the same frequency, and this is affecting your signal strength.

There are a couple of things you can try. There are third-party apps that can scan the wifi spectrum and identify signal strength and interference levels. https://www.intuitibits.com/products/wifiexplorer/ is one example (with a 3-day free trial which should be long enough to see if it identifies your problem).

Another option (if supported by your router) is to temporarily configure your base station to use different SSIDs for the 5GHz and 2.4GHz networks. This will let you force the Mac to use one or the other. You can then join each one manually and get an idea of the signal strength (you can use an app like https://www.speedtest.net/ to get a comparison on the different networks.

Once you can identify the problem you can find the solution. It may simply be repositioning the base station, or replacing the base station with one that has a stronger radio transceiver (or a mesh network of multiple base stations). It may be a simple configuration change (e.g. force the 5GHz network to an unused band that your neighbors aren't using). Lots of options... just need to know the cause first.

Wi- Fi Radio Signals from your router fall off as the cube of the distance if you have clear line of sight and no competition from neighbors' Routers or known interference producers like microwave ovens.

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

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

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 network. Screenshot, transcribe, or photograph the results and post back in a reply on the forums. Looks like this older one.

Just create a 5Ghz only band on your WiFi router and connect to that one using your Apple devices. The brand of your router may call it Band Steering or Frequency Splitting and how you do that will depend on your router. Yes, the distance will be shorter than 2.4Ghz, but the bandwidth is greater with less interference.

If your Mac isn't connecting to the internet over Wi-Fi - Apple Support explains how to use Wireless Diagnostics.

I’ll assume your ISP has informed you that the whole point of using 5 GHz and 6 GHz networks is shorter range and reduced structural penetration. That is part of what makes denser Wi-Fi networks possible. Using 2.4 GHz has better penetration, but has its own issues and not the least of which is interference.

What did the ISP already suggest doing? Presumably, that included testing the MacBook Pro Wi-Fi connection on another Wi-Fi network, and probably also establishing better wired or mesh coverage for your home network? Anything else? Any findings with that?

What is your particular Wi-Fi gear vendor and model?