Using custom background when on someone else's meeting

[inctrl]

An organization I belong to uses MS Teams for meetings.  I can join their meetings when I get an invitation, by clicking on the link they sent, but there's no option, while I'm in their meeting, for me to change my background.  I also have a free account with MS Teams, which says it allows for a custom background, but I can't find a way to join the other meeting while signed in to my account.

Is there a way to activate the custom background while I'm in a meeting that someone else started, either while I'm signed in to my free account or not?

[#2]

In theory, if you are using the Teams Desktop client to attend a meeting, you should be able to change your backgroud.

[#3]

I have the same problem, if I enter an externally organized meeting I cannot change the background nor apply the effects, in both cases I don't see anything in the list.
For display effects, I have checked and the processor supports them and I have a geforce, I have the updated version of teams desktop