Our forum is mainly focused on the general issue of Skype for Business troubleshooting. According to your description, your issue is not in our support scope. The following suggestion is just for your reference.
To my knowledge, you could choose to add your own domain or the default domain(onmicosoft.com) when going from 365 personal to Business. The following blog show the detailed steps:
https://www.mdsny.com/step-by-step-guide-to-set-up-o365-for-business/
Note: Microsoft is providing this information as a convenience to you. The sites are not controlled by Microsoft. Microsoft cannot make any representations regarding the quality, safety, or suitability of any software or information found there. Please make sure that you completely understand the risk before retrieving any suggestions from the above link.
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