Headset link:
https://steelseries.com/gaming-headsets/arctis-7
Correct?
Does the following link present the Product Information Guide that matches your headset?
https://downloads.steelseriescdn.com/guides/Arctis7_Refresh_PIG_04.pdf
If so, Page 8 indicates that a red light is a low battery condition. Even lower if blinking fast.
Are you sure that the battery is being properly charged? Put the headset into charge and doublecheck all connections.
Other things to do:
Plug the headset, transmitter etc. as normal.
Then gently wiggle, pull, and twist the connecting cables especially at the ends and plugs.
Watch if the red light goes out and/or your mic audio begins to work - even briefly. Or perhaps begins to charge.
If so there is likely a broken wire within the cable or a bad connection inside the mic itself or its plug.