Users can't join external Microsoft Teams meetings

Apologies if this isn't posted in the right spot, but as you can imagine the search terms return a lot of similar sounding, but irrelevant results. Our organization ONLY has M365 Business Apps licenses. No Teams. No Exchange, etc. When we attempt to join an external meeting as a guest through the browser, we get a couple different errors. Sometimes the error reads that Microsoft Teams is disabled. Sometimes it's just a generic "We've run into an issue" error. However, if we try and join using a browser that does *not* use seamless SSO, then we're able to connect to the host's Teams session without error. I've done some poking around, but because we're not a 100% MS shop, I'm not entirely sure where to look in the admin portal to resolve something like this. I don't see any specific settings that are disabling Teams, or to allow users to connect to external meetings. However, something in our tenant is obviously stopping users from being able to join. Any pointers would be greatly appreciated!