The "read receipt" is a red herring. I often click on newly received instant messages on my phone in situations where I am unable to respond. This triggers a read receipt, but it does not mean that (1) I was able to fully read the message, (2) that I had time to think about the issue or (3) that I had time to deal with the issue and reply.
Since my phone does not offer an easy way to mark the message as "unread" or copy it to some kind of To-Do list, I can only attempt to remember it until I have time to deal with it. This attempt usually fails. Thus, if you need me to do something, sending me an instant message is not a reliable way to ensure that I do it.
On the other hand, sending me an e-mail is reliable: The mail stays in my Inbox until I dealt with it in some way (e.g., by replying, if it's an easy task, or by creating a To-Do entry, if it's a hard task).
To make a long story short: You need to find out what communication medium works to get your teacher to process your request. Note that this might be different from the medium that the teacher told you to use.
You tried instant messaging (multiple times), and it failed. So you need to try something different. If your issue isn't urgent, you could try e-mail next. This works for some people, but it won't work for others. Unfortunately, some people have poor task management skills, and, for them, asynchronous communication does not work at all.
In that case, it's time to switch to synchronous communication. Pick up the phone. If people don't respond to a message after a reasonable amount of time, it's perfectly fine to follow up with a phone call.
If you are anxious about calling them in person, here's a script you could use:
You: Hi, I'm Lou, one of the students from your remote course on basket weaving.
Teacher: Hi Lou, what can I do for you?
You: I sent you a Teams message a few days ago with a question about frobnicating foos.
At that point, you can just stop talking and let the few seconds of uncomfortable silence work its magic, after which the teacher will pick up the conversation, apologize for not responding earlier and answer your questions. If uncomfortable silence makes you ...uncomfortable, you can ask whether they received your message instead.
After your issue has been resolved, it might be a good idea to ask how they would like to be contacted in the future (Teams, e-mail, phone, something else) for questions relating to the course.