Direct connection from RSS TX Out audio to the FT450D front panel RJ45 microphone jack would cause distorted audio overload when the PTT closed to TX. making it unusable.
I bought the MFJ 5706MD cable and pin switch board to use the 6 pin din data output for TX and RX audio. I was not happy with the various issues that affected the audio and mode switching problems.
I now have perfect TX audio with this simple circuit setup using the front panel microphone RJ45 jack. and not needing the data port or the mfj 5706md for RSS operation.
Locate MIC GRD contact #4 from top of RJ45 on ft450 and then GND conrtact #7 and connect them together then connect them to one lead of a 50 V .1uF ceramic disc capacitor (mouser part # 140-50v5-104z-tb-rc)
Them connect other cap. lead to RSS TX out GND with audio 3,5mm audio cable.
Locate MIC on ft450 RJ45 jack connect to one lead of a 100K ohm single turn potentiometer 10% (mouser part # 652-3362p1-104lf)
Them connect single lead to RSS TX out center pin. adjust for 72K ohms.
Audio speaker icon on desktop home screen left click and adjust for 60 t0 70 %
i now have clean adjustable Microphone input even when using my iphone LAN or WAN
Good luck with this arrangment its working for me.