Mail Sub-Folders Not Showing on iPhone 10s

OK....I've read numerous posts on this subject, but am still confused!!!

I have several email accounts with PTD (PennTeleData), and one with Yahoo. I'm not certain if the accounts are POP or something else.

Only a few of the PTD accounts' sub-folders I've created on-line and/or with Win. 10 Outlook show on my iPhone. While I can add sub-folders to the PTD accounts on-line and/or via Outlook, they do not show on the iPhone. Similarly, I cannot create sub-folders for these PTD accounts on the iPhone.

HOWEVER, all the sub-folders I've created for the Yahoo mail account do show on the iPhone, and I have no problem adding sub-folders to the Yahoo account via the iPhone.

If the problem is with the "type" of email accounts I have with PTD, how can I resolve that issue? PTD has been around for a long time, and seems to be "up-to-date" regarding their products.

You help is greatly appreciated! Thank you in advance!!!