Hello and welcome to Microsoft Q&A @Kenny, Craig (EthosEnergy) .
So, do I understand correctly, that your authentication alone takes 90 seconds, on average, no matter what you connect with? I can certainly see how this would cause a timeout in Synapse and many other applications.
I ask, because taking more than a minute to authenticate is, well, so far beyond the norm as to be unreasonable in all the cases I can think of. I am not blaming you or anything, but I am concerned. I could look into whether there is a way to increase connection timeout, but I worry that your authentication provider has something bad happening, and ignoring the cause might make it worse.
Imagine trying to log into your email, and waiting 2 minutes before even getting to load. As an end user, I would have pressed reload button after 25 seconds, thinking something broke.
Modern applications would have done multiple retries by 30 seconds, for comparison.