Connman

ConnMan (short for Connection Manager) is a network management service created by Intel, built with embedded use cases in mind. It is shipped by default on some desktop and mobile distributions however, such as SailfishOS.

USE flags
Make sure to enable the proper USE flags for what you plan to use ConnMan for. If this is a mobile device and you want mobile data support, make sure to enable the  USE flag.

Enable Startup on Boot
Note that having multiple programs running at the same time that manage networks (networkmanager, dhcpcd, etc) is not a good idea. However, ConnMan can be configured to not touch WiFi/Ethernet so it can play nice with these other programs, see the bottom of this wiki.

OpenRC
To start ConnMan at boot:.

systemd
placeholder, if you're a systemd user please add your insight :)

Using ConnMan
is an interactive CLI application that comes installed with ConnMan. Make sure  has been started, either manually or through the init system, or ConnMan will be unable to function.

can be launched with no arguments to enter interactive mode, or a command can be specified after it (useful for scripting!)

Listing all available Technologies
Should list the available networking technologies for ConnMan, such as  (WiFi support). If you don't see the technology you're looking for, make sure it was set in ConnMan's USE flags, and that support for the network interface is built into the device's kernel.

Powering On the Chosen Technology
Replace  with whatever technology desired.

Scanning Wireless Networks
"Scan completed for wifi" should return when scanning is done.

Showing Available Services
This outputs all available services, no matter if WiFi, Ethernet, or other. If the device is connected to the network,  is in the leftmost column, followed by the SSID in the next, and then followed by a unique identifier. This unique identifier is needed to connect to the network.

Connect to a Network
Note: Tab completions are functional in interactive mode!

Fill in the unique identifier from what is after your network name from. If connecting to a wired or unsecured network, no further action is needed. If connecting to a network with a passphrase, enter it when prompted. ''Note: Double check your password! ConnMan can often be annoying and not re-prompt for incorrect passwords.''

Testing your Connection
To reveal the connection to the Local Area Network (LAN):

And of course to test connection to the internet:

ConnMan is throwing some cryptic error I can't find a reference to online!
It's most likely lack of kernel support for the device's WiFi card. Make sure if built as a module it's loaded and that  isn't complaining about a load error.

You can also try using Gentoo's distribution kernel to see if that gets your network card going. If so, run  with and without the distribution kernel and   the results to view the kernel drivers that were in use with the distribution kernel that weren't with whichever you were running originally.

How to use ConnMan along with NetworkManager/dhcpcd?
Obviously, change the interface names to those that you wish to blacklist with ConnMan. This allows another service to manage those connections, while utilizing ConnMan for other network interfaces (ie, ofono, bluetooth). If connected via SSH to your device, this should be done first to prevent ConnMan closing the SSH tunnel.