Issues connecting phone to radio

We recently purchased a 2011 Citroen C4 Picasso VTR+ HDI S-A with a continental RD4 stereo fitted , Which we are struggling with connecting any media to - the unit plays as expected as do cd’s when inserted , however ..........

plugging in a usb stick with music files on results in absolutely nothing happening .

plugging phone in via an aux lead results in the display showing aux 1 - but no sound coming through the radio .

plugging in via bluetooth results in the display showing the phone as connected & I can scroll through the contacts visible on the display - I can even click to begin a call . Yet even though the receiving phone will ring & they can answer - the radio station fails to mute & I get no audio etc from the call .

I paid to have a diagnostic done but they couldn’t pinpoint the issue “it could be the usb / aux port or the radio unit itself or a wiring issue” extremely helpful - NOT . I have replaced the usb / aux unit as that was only about £20 & it made no difference whatsoever (the lead & my phone work fine through the aux input on my daughters 2007 Honda) .

i am using an iPhone 8 running iOS 13.3 and wondered if anyone had any suggestions / advice - the bizarre thing is - my wife has the same phone - but the car won’t even find hers during a search (tried several times)

any helpful info appreciated