Yes, they can terminate on behalf of you because you gave them permission. Didn't you ask to switch to Maxis?

Why suddenly you upset.

I believe the case if that there are not enough ports in your area. Thus, in order for Maxis to install their service, you need to terminate Unifi first to free up the port. Only then will there be one space for Maxis to your house.

So if you install your Maxis (or are going to), i.e. Maxis already book the port, then of course you cannot install back Unifi. Because that last remaining port was taken by you (under Maxis).

Is Maxis still proceeding with the installation?

They are probably dealing with the port transfer/ protocol etc. Just wait 7 days (sucks, I know), after which you can really go after them