Unable to send iMessages using WIFI only (phone does not have cell service)

The iPhone model I am using is a 2020SE. I am trying to set it up for someone who is blind (and a

VoiceOver user) and does not have the resources to buy a cellular plan.

However, she has WIFI access at home. To enable her to make and receive text

messages and to make phone calls over the Internet I downloaded the free

"Talkatone" App. That app is a bit difficult to use via Voiceover but

it's working. However, most of her friends and family have iPhones. So,

I thought she could send text messages as iMessages to them using her WiFi

connection because that will be easy for her to do via VoiceOver and Siri.

However, every time I try sending an iMessage from her phone to my iPhone using

my home WIFi, I get notified that the message was "Not delivered".

iMessages are enabled in the Messages settings, I checked on that. However, it

says "Waiting for activiation" underneath, but I cannot click on that

to perform any actions. I am a bit stumped by this. I travel oversees every

year and always take my iPhone (the old SE model) with me. I do not buy any SIM

cards for oversees cell phone carriers so I don't have a cell data plan when I

am over there. I am always able to send iMessages to friends in the US who have

an iPhone using WiFi. So, why can't I use WiFI with this phone that also does

not have a cell carrier associated with it? If you had any ideas how to solve

this problem, I would be very grateful. Thank you!