It's Possible you use the Keys . or , on your Keybinds? You can use this Buttons to change the CB Channel on TMP.
Keybinds:
[,] - Used to change to a lower CB channel.
[.] - Used to change to a higher CB channel.
[Y] - Used to open the chat menu, use [ENTER] to send your message.
[X] - Used to activate the CB Radio (voice chat), release to stop talking.
[TAB] - Used to show the nearby players, report, settings and previous players.
[F9] - Used to disable chat.
[F11] - Used to disable nametags and the [TAB] menu.
https://truckersmp.com/knowledge-base/article/85