Android Outlook using outlook proxy email address for primary account, can't receive emails

Longtime, fairly contented user of Outlook in Microsoft 365 here. 

Yesterday I tried to set up Outlook on my Samsung phone.  I was not able to start by incorporating my primary email account, based at my ISP, even though Outlook spotted that account and offered to include it.  (I was confused about the proper IMAP port number.)  I then took the route of bringing in a gmail account used as a secondary.  That worked well.  However, Microsoft required me to set up an outlook.com email.  Now my phone Outlook nominally has my ISP account as primary, but when I begin a new message, it lists my outlook.com address and my gmail address as sending options, not my ISP address.  Emails I send from the account show as having  been sent from the outlook.com account -- mailto:email address removed for privacy reasons --, although when I reply to them, my correct ISP address shows up as the prospective recipient in the email draft.

I might be able to put up with this -- not sure about people I correspond with, however -- but the ISP account on my phone outlook is not receiving any emails.  (Oddly, there is one email from 2015 from the Outlook.com team about getting started.)  I can't find a way to adjust what I assume might be incorrect settings for the ISP server.  I'm wondering if I should uninstall the current phone Outlook and try again.  

A side question: what is the purpose of requiring me to set up an outlook.com email?

Thanks for any help you can provide.