The following post is a rant, but it may help those struggling with MySudo on GOS.
I am a fellow MySudo user that also has run into the same issue you've encountered. Nothing I do will get MySudo to work on GOS. So, I sent a rather long and detailed email to MySudo Support that included the status of just about every setting and toggle on my phone that directly and indirectly relates to the MySudo app to see if they could provide any additional help. I was hoping they would forward the email to the engineers. However, although their reply was not rude, it was very dismissive, and developing MySudo for GOS users appears to be very low on their priority list.
It makes sense, though, because it appears MySudo's target audience is those who wish to have a modicum of privacy and identity compartmentalization, along with the bells and whistles and conveniences of stock OS's like iOS and Stock Android. After all, MySudo is a proprietary and closed-source app that relies on Google to deliver a functioning app to its customers. Not really privacy-first.
For brevity's sake, I'm not going to delve into all the different settings and combinations I played with to get MySudo to work again on my GOS device. None were successful for me. However, some people have gotten it to work. So, it is a case-by-case scenario. [deleted] This post here may be of help to you.
I've been a paid subscriber of their Max plan for over a year, but I will not renew it. I've already transitioned to JMP.chat, and it's much better. JMP.chat is cheaper, extensible, and open-source, relies on the standardized Jabber network based on the XMPP protocol, and their support is fantastic!
In my opinion, MySudo is a valuable privacy tool that can help many people, like those seeking a "just-works" approach to VoIP and broader identity compartmentalization. However, those who wish to adopt a DIY and more robust approach to privacy (like flashing GOS on a Pixel) are better served by tools like JMP, IMHO.