If it’s a valid 3rd party certificate (and not working out-of-the-box) then you could I guess add in there the root CA certificate instead (the one that is top-most/self-signed in that certificate chain). But that is not relevant to why it didn’t work… it should have worked with adding your own cert. directly as well.
Can I try to access that URL or is it private? I remember doing what you did a couple of years back and then it worked for me, but I didn’t try it again since then.