Most photographers likely had to deal with this at some point.  Stuck polarizer filters.  Internet is full of advises, including many YouTube videos.  As dominantly nature photographer I also had to deal with it several times and was able to unscrew it using one of these methods.   Until only inner ring got stuck - not entire filter


[Inner Polarizer Ring stuck on top of Camera Lens]


[Polarizer Filter left and inner Ring right]

Anything I tried didn't work.  It was stuck good.   Even apparently bulletproof method when everything else fails - hose clamp - didn't help;  just couldn't get enough grip


[Hose Clamp - photo for illustration.  If you try this one use rubber gloves for grip and separate lens from camera first]

Next step - took it to several camera stores that had repair shops.  At one place they had special machine that is used for cases like this.   Tried - no success.  Rest wouldn't even touch it afraid not to damage the lens; gave me advises instead of which I was already aware of.     I was ready to give up.   Then one afternoon I met photographer on photo shoot in local park.  I chatted with him and he told me he had same problem and was able to fix it very quickly!  How?  Pliers. Believe or not.   Grab and squeeze ring, then twist it couple of times.  I didn't think it would work, but I gave it a try.  Instead of pliers I used adjustable wrench


[Adjustable Wrench I used instead of Pliers]


[Get a good hold, then twist off at couple of places (This shot was taken later after ring was already off, you can see on top where I was able to rip it)]


[Within 10 seconds ring came off!]

You need to be careful not to damage the lens - always take lens off camera first (photos above are for illustration).  Sending lens to Canon would solve the problem;  they would saw it off, but would charge you dearly.  DIY is always the best option if possible.

 
Hopefully this can help someone.

They make plastic filter wrenches in different sizes. Using one doesn't bend your filter ring and gives you just enough purchase to get a stuck filter off. Many years ago I shot a bunch of night images with a polarizing filter on because it was stuck, went to my local camera shop (when those were still around) and got one since then I always keep one in my camera bag.

I've got a large & small plastic one that have lasted many years, but the rubber coated metal ones look even better:

https://www.bhphotovideo.com/c/accessories/Filter-Wrenches-for-Formatt_BF_48_5_CLRSU/616434-REG-99075

I think pliers was how I got a filter ring off a lens I dropped. It was not so easy, but I did get it off and the lens was ok. The filter was toast - but the photos taken with a shattered filter were surprisingly good - probably wouldn't pass microstock review at the time though.

Pliers once saved my lens too (after trying a filter wrench). I was lucky that the filter is still usable.