The U4B has a voltage regulator on the plus and minus terminal?
Never exceed 5.5v on the plus and minus and it will start when powered.
Take the 5.2v from your solar usb rig power and apply it to plus and minus and it should start.
Sensing power on the Usb line will cause the U4B to make other things happen that will start a terminal application that you probably do not want running if you are just transmitting to an antenna with the U4B.
Dave VE3KCL
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VE3KCL