Hi my problem was Bigpond email. I have had this problem with Telstra and other companies when there invoices etc wouldn't send to me after their company upgrades. The solution was for that company to manually type my email address and send me an email from their system (not picked up auto) and thereafter all was back to normal.
Unfortunately this could not be done from MYOBs system and may not have even worked in this case. Like you I couldn't get access to change the 2FA method.
I had to ring them and patiently wait to have my 2FA changed to SMS. The person I spoke to was very helpful. He said it was a Bigpond issue. I did not want to use SMS but had no choice. The Bigpond emails took so long they always timed out when applied. He gave me a code to copy and keep so if my phone was forgotten I could use the back up code.
I too lost a lot of work time sorting it out so hope this helps.