I had an issue similar to this a while back synching my library across Apple devices. If you have an older iTunes ID, there’s a good chance it’s different from your Apple ID.
First, check the settings on your iPhone to see if your ID being used for purchases is the same as your Apple ID (Settings > your name > Media & Purchases). It will probably show you your iTunes ID and ask for that password. If it’s different from your Apple ID, the Media & Purchases ID is the one you need to use to log into Apple Music in the WiiM Home app, and you should then see your entire library.
I never did figure out how to bring my iTunes purchases under my Apple ID, which would make things so much easier dealing with media and anything related to the App Store. Probably needs a lengthy phone call with Apple support. But I’ll bet a dollar that’s your problem.