The short answer:
In your terminal:
$ sudo nmcli connection add type wifi con-name <new_connection_name> ifname wlan0 ssid <new_SSID>
When you're "in range" of your new wifi connection:
$ nmcli connection up <new_connection_name>
A longer, rambling, opinionated answer:
Sadly, we are now stuck with network manager (nm) as our default; wpa_supplicant.conf is gone (subsumed by nm). This is due to decisions made by "The Raspberries" just before the release of bookworm. FWIW, network manager came from the same place that systemd came from - that may help you understand the mindset at work behind the design.
EDIT:
If you're interested in alternatives to nm, there are some listed in the Debian wiki. I'll comment on two of them here:
systemd-networkd: There is an excellent group of Q&A here on RPi SE related to systemd-networkd (mostly fm @Ingo), but they are somewhat dated now - mostly 'buster'-vintage. I do not know if they are still applicable or not.dhcpcd: Still available through apt, but dhcpcd5 (the default just prior to 'bookworm') is now labeled as a "dummy transitional package". There's a forum post on using it that seems fairly thorough. However, recall that RPi didn't follow the upstream version of dhcpcd for many years - they were "stuck" in version 5 (for reasons that are unclear) until 'bookworm' arrived.
Also note that none of the alternatives are supported with "official" documentation. That leaves you more or less "on your own" as far as configuring them for use on your RPi!
This edit was added to address a comment from our Moderator - who insists there are alternatives. I'll leave it to him to show us if they are practical alternatives or not.
Personally, I don't like network manager (nm); I think it's arcane and awkward to use, but I still use it. I'll continue hoping that something better will come along, but until that happens it seems that we'll have to learn enough nm to get by.
With that in mind, you might try the following:
List the nm "connections" on your system:
$ nmcli connection show
... on my system, here's what I see:
NAME UUID TYPE DEVICE
preconfigured 4564bc40-6546-494e-847b-7efdfe918d60 wifi wlan0
lo d9a67218-2f52-4d6c-a2d3-87ee0f60d161 loopback lo
Wired connection 1 8282b463-5561-3ae6-a00d-34c201b3a8d1 ethernet --
You've ruled out ethernet, and loopback isn't any help here, so we're left with preconfigured as the actual (bizarre) connection name. (No doubt this name was chosen by some really intelligent people.)
We know that preconfigured uses wifi (DEVICE=wlan0), so our approach will be to simply duplicate preconfigured to add another connection for a second wifi network.
As it turns out, the actual configuration file(s) are located in the folder /etc/NetworkManager/system-connections. We can view it:
$ ls -lA /etc/NetworkManager/system-connections
-rw------- 1 root root 262 May 27 2024 preconfigured.nmconnection
$ sudo cat preconfigured.nmconnection
[connection]
id=preconfigured
uuid=4564bc40-6546-494e-847b-7efdfe918d60
type=wifi
timestamp=1716798784
[wifi]
mode=infrastructure
ssid=Mesh01
[wifi-security]
key-mgmt=wpa-psk
psk=nofyfb
[ipv4]
method=auto
[ipv6]
addr-gen-mode=default
method=auto
[proxy]
Now we know what a properly-configured nm wifi connection looks like! If you're interested, you can view all the gory details of the connection as follows:
$ nmcli connection show "preconfigured"
Which provides about 175 lines of brain-numbing crap! But some of it may prove useful, so at least know it's there.
So - on with creating a new nm connection for a 2nd wifi network
It might be possible to simply make a copy of preconfigured.nmconnection file, generate a UUID and substitute the values for another wifi connection. But instead, let's ask nm to do that for us:
$ sudo nmcli connection add type wifi con-name "preconfigured2" ifname wlan0 ssid "Mesh01"
Note that I decided to call this connection "preconfigured2", and used the same SSID. You could change either or both of these to suit your needs/desires.
Afterwards, we now find a new file has been added in folder /etc/NetworkManager/system-connections. Note that it is essentially the same as the preconfigured connection without the password/authentication section:
$ ls -lA /etc/NetworkManager/system-connections
-rw------- 1 root root 215 Jan 4 09:20 preconfigured2-aa3e2db6-d295-4c28-a8e5-51ea4d48e042.nmconnection
-rw------- 1 root root 262 May 27 2024 preconfigured.nmconnection
$ sudo cat preconfigured2-aa3e2db6-d295-4c28-a8e5-51ea4d48e042.nmconnection
[connection]
id=preconfigured2
uuid=aa3e2db6-d295-4c28-a8e5-51ea4d48e042
type=wifi
interface-name=wlan0
[wifi]
mode=infrastructure
ssid=Mesh01
[ipv4]
method=auto
[ipv6]
addr-gen-mode=default
method=auto
[proxy]
Let's use nmcli to show our connections again:
$ nmcli connection show
NAME UUID TYPE DEVICE
preconfigured 4564bc40-6546-494e-847b-7efdfe918d60 wifi wlan0
lo 37ad01d0-d685-4949-81f4-0e532ded0721 loopback lo
preconfigured2 aa3e2db6-d295-4c28-a8e5-51ea4d48e042 wifi --
Wired connection 1 8282b463-5561-3ae6-a00d-34c201b3a8d1 ethernet --
And so we now have a new nm connection that we can use to connect to a new/different wifi network. To make the connection:
$ nmcli connection up preconfigured2
