This is the 9238th most frequent Russian word.
соединенным
"Соединенным" means "united" or "connected."
Here, 'соединены' is the past participle of 'соединить', emphasizing the state of being connected.
Эти города соединены железной дорогой.
These cities are connected by a railway.
In this context, 'соединенным' is an adjective derived from the past participle, describing the device's connection status.
Устройство было соединенным с компьютером.
The device was connected to the computer.
Here, 'соединенным' modifies 'устройство', depicting the state of connection as an attribute of the device.