Just to clarify (as the subject title is misleading in regards to my query) - I am able to send/receive emails on my mobile phone, but I am unable to log on to the online email on a PC.
The fact that it no longer works in a web browser but does on an email client, is a bit of a worry. When a mailbox is in the process of being closed down and deleted, often the first thing to go is webmail access, for various and boring technical reasons. Followed sometime later by access via any email client.
I have a bad feeling that your email was never actually transferred across to the business account and as such is orphaned and in danger of being closed down without further warning, the 90 day business is more of an aspiration rather than a realistic hard deadline. Just to add to this, this forum is geared to domestic users and the ability of any of the forum team members to intervene and check is severely limited. You may well have to contact the VM business support number to get any help on this.
But if I were you, I think I would prepare for the worse and backup any and all emails you want to keep while you still can, just in case.