It’s against the law for someone to sell private health insurance on the Health Insurance Marketplace to you if you have Medicare.
A lot of therapy places that actually help elderly people gain function, do not accept Medicare plans regardless if you have a “supplemental” plan through a private insurance company.
True “secondary” plans are the only way to go but you can’t get them unless you are actively working (as if they didn’t work for 45+ years of their lives already).
Either pay providers an acceptable amount so they can be contracted with Medicare or ban this requirement.