Unstable Internet on Zoom calls

It sounds like you are doing all of the "reasonable" trial and error steps. And trying Wired versus Wi-Fi is another perspective.

I too run Skype and Microsoft Teams and for both of those apps, the sending audio worked fine. So I was dubious that I truly had an "unstable internet" issue. It was only my Zoom app that was cutting off my sending audio. Restarting my router after having SIP ALG disabled (my QOS was already disabled) did the trick for me. 

I see 3 things that might conflict Zoom + Router; Zoom + Firewall; or Zoom + Firewall + Router.  Also, if you use VPN to reach your company and then use your business Zoom account then 4th option is Zoom + Router + VPN. This is my simple perspective.

I ruled out firewall first ... by testing via my company's VPN (which would bypass my local firewall). If the same behaviour occured with or without using VPN, then it was not my firewall.

These are some of the things that I did to "isolate" the issue and figure out that I had a conflict between Zoom and my Router. You might have a different conflict.

My steps:

1. Uninstall Zoom client and use the Zoom web version to attend a call.

Do you still have audio sending problems or does the sending audio work?

For me, my sending audio worked just fine in Zoom via the web. And I stopped seeing the "internet unable" message. Of course, using the web version of Zoom limits your functionality so this is NOT a fix just a test to help isolate the problem.

Thus, Zoom web version works just fine (and I did not have "unstable internet" issue). If my internet is good enough for Zoom web version than it is likely that my internet is just fine for my Zoom client. So I stopped focusing on having a bandwidth issue and worrying about using wired versus wi-fi.

Whether I used my VPN or not, my sending audio in Zoom web worked.

Thus, not a VPN issue.

2. Next, I reinstalled Zoom on my Windows machine. My sending audio immediately stopped working. My Zoom connection had a near 100% packet loss. Whether I used my VPN or not, my sending audio would not work.

Thus, Zoom Windows client was definitely an issue.

Thus, VPN was still not an issue.

3. Using my phone as a hotspot (versus my router).

I connected my laptop to my phone hotspot and tested the Zoom Windows PC client, and I was shocked to see my sending audio WORKED! Whether I used my VPN or not, my sending audio always worked.

Thus, my Router was in conflict with Zoom client (or vice versa).

This is when I started checking for all of the "audio voice optimization" options that my Router supported and started turning them off one by one. Your Router may have other options than what mine had.

Good luck!