Message+ activation not working

I just bought a new Pixel 5a and after transferring over all the files/contacts and the sim card out of my old pixel 3, I am unable to receive messages through message+. I can send messages but cannot receive them, unless I take out the sim card from the pixel 5a and put it back into the pixel 3. Message+ on the pixel three is already activated, but when I try to activate it on the pixel 5a, it brings up a loading screen for a brief moment then disappears, not allowing me to activate it. 

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Re: Message+ activation not working
Customer Service Rep

I can definitely help you with transferring your content to another Pixel - is your previous Pixel off, and does your new one have Wi-Fi Disabled? *Stephannie