Howdy,
The IT team who setup Nextcloud at my organization is no longer with my company. They took zero notes and left zero documentation on any/all projects or applications. One of my companies uses your product and it is having issues with the mobile portions and users opening viewing documents.
I have no admin accounts or details on recovery to access: Nextcloud Portal - Members Login
How do I go about verifying my identity to replace who ever/whatever was attached to the support portal access?
Are you sure that there were even people who used Nextcloud GmbH support via the portal? You have to distinguish between the free software and the support e.g. here in https://help.nextcloud.com and the commercial support on the page https://portal.nextcloud.com you mentioned. For commercial support you normally pay money each Nextcloud user. Have you paid for it (to Nextcloud GmbH not your IT team)?
But you can possibly ask there if your Nextcloud or the users you mentioned are or were known.
Enterprise - Nextcloud
Pricing - Nextcloud
FAQ - Nextcloud
The userbase for the nextcloud are a bunch of flooring installers. It took two weeks of me meeting with them to even understand what this product was. I just cracked access to the server itself and no documentation was left of the server either. I am still waiting for users to provide the actual portal access they use for the product also.
I think the “flooring installers” only use the Nextcloud server itself e.g. https.//cloud.server.tld and never had an account on any external portal.
You can only wait for the “IT team”. Maybe they have had an account at the portal or maybe they didn’t. Nobody needs there an account to install, configure and manage Nextcloud. Nextcloud is AGPL (free software). You pay at Nextcloud GmbH only for the support.
Sadly I am the IT team. Last group was ill prepared and under educated. I was brought on with my team to get this company and its divisions to a corporate level.
Thank you @devnull for the clarification on support.
You also can search IT spcialists for Nextcloud. There will certainly be experts in your area. Maybe it is also an option to migrate your actual Nextcloud from on-prem (in your company) to a hoster data center. Microsoft wouldn’t offer you on-prem with Office 365 either. Also emails are not usually hosted on-prem these days either. 
But at least with Nextcloud you have the option.