Mirroring to av input?

[Beachin22]

I'd like to simple mirror my S9 to an A/V touchscreen car stereo with only a 3.5 av input. Head unit is a jvc kw-v250bt. It doesn't support MirrorLink or apps that use MHL.
Supposedly it will with hdmi converter but it signal on screen is scrambled. Phone recognizes hdmi connection.
Am I missing a setting here? I have hdmi setting to mirror instead of dex but neither works...