I have an old Win XP laptop which has wifi in it. I would like to convert this laptop to a wifi hotspot. I should be able to connect from both phones as well as computers. Is there any in-built mechanism to do this within XP?
Windows – Making a Win XP laptop a Wifi Hotspot
Get a WiFi extender or a client bridge device (something that can connect to WiFi and allows connections via an ethernet port), and connect it to your phone's hotspot.
Then, disable the access point (repeating/extending function) on the WiFi extender and plug an ethernet cable from a LAN port on the extender to the WAN port of a router.
You can usually find old routers on Craigslist for about $15 to $25. Connect all your devices to the access point broadcasted from the router, and that should do. The router, is given one IP, and then splits it up for other devices.
WiFi extenders do not have routing functionality. The reason for this, is because it would be more expensive to build it that way, and... Traditionally it would cause less issues because it connects to one LAN network pool. Aka, the devices on the main access point can still initiate connections with devices connected to the repeater.
My laptop with Windows 10 can connect to 2.4G WiFi as well as 5G WiFi, but when I turn on the hotspot in Windows 10 settings, it says that only the devices supporting 5G WiFi can find the hotspot.
Your adapter is dual-band which means you have to configure Windows so it knows which band you want to use (2.4GHz or 5.0GHz) for the virtual router. This configuration might be at the device level, which means you want to look at the configuration of the device in Device Manager. If you still can't figure it out there are multiple third-party programs that will provide you that granularity configuration of your network.
However if you are connected to your network using the 2.4GHz band, and you're using the adapter to create a virtual hotspot, that would explain the reason you can only create an 802.11n 5.0 GHz hotspot. You can't create a virtual network on the same band you are connected, to an existing network on given your current hardware.
The reason you were able to do it with the other adapter is likely because the configuration of that adapter allows you to configure both bands to the same frequency. I can only use my two decades worth of engineering experience to make an educated guess, considering, I know nothing about the other adapter. Given how this feature works in the first place I am confident that I am right, connected to your network using the 5.0GHz instead, and you will be able to create a virtual network on the 2.4GHz band.
I am wondering whether it is my wireless adapter or Windows 10 that limits 5G WiFi broadcasting.
What you describe is the limitation of the adaptor. Windows does not have a limitation.
Why the adapter works in this way?
It was designed this way.
Assuming you wanted an inbuilt mechanism to convert to a wi-fi hotspot, you could use the Wireless Network Wizard. However, and if I know Microsoft, they might over-complicate things. Also, I never used the tool, so my description might be wrong.
As far as I know, this is the only inbuilt mechanism in Windows XP.