Whenever you have cross-posted stuff on different instances, you find yourself unable to comment or upvote on posts.
This is where Azzurite's script comes to rescue you.
How to get it working?
I'm will be using Iceraven but you can also use it on standard Firefox for android with add-on called Tampermonkey which was up to version 2.9 open source but all newer versions are distributed under a proprietary license.
Next step is to install Violentmonkey (open source, that's what I chose). Go to burger menu on Firefox, click on Add-ons/Add-ons Manager look for Violentmonkey (or Tampermonkey). Install it.
Next visit this website click on "Install this script" you will be automatically redirected, click there on "Install" button.
Next go to burger menu, click on Add-ons/Add-ons Manager/Monkey addon/Settings under installed scripts (if everything went ok) you'll see that script was installed.
Now when you visit cross-posted content on different instances you'll find in upper right corner link saying "Show on ________" which is your instance you're logged in or will be asked if you want to redirect to your home instance. After that you can comment & upvote on different instances.
Link for dev here