Dave, I should have asked if you know what version of the firmware you have? If you have v1.79b02 (14th October 2022) or newer, the "line" function is automatically enabled when the radio senses that the correct cabling is attached, ( back to my Digirig recommendation).
?
Again from the manual, "New data mode: U-D; L-D, for USB DIG; LSB DIG£¬
In data mode: ACC port LINE IN is selected as audio in and the hand
mic is muted; LINE OUT via the ACC socket is enabled? "
Bill KD9TWA