Don't use any glue If the glass is separating there must be a reason for it. Usually it is puffy battery. If you can see inside check the battery. If battery is good, put the glass back on, heat the back glass around the edges with hair dryer, use plastic clamp or rubber band to keep the glass in place, leave the phone for an hour
Thank you for reaching out. I can certainly understand your concern with your rear glass coming off your Note 20 Ultra. Please know that self repair could void your warranty. If you would like to set your device up for service, Please feel free to reach out to us directly via one of the following options and include a link to this thread:
I agree with TheGalaxy's statement that if the back glass is coming off, there is a reason and it's most likely the battery swelling but if you check and replace the battery you can most definitely reattach the rear glass panel. I'm a big advocate for the right to repair your own products. The best way to reattach is the OEM method, buying an adhesive replacement strip that's perfectly cut to the phones dimensions. A quick Google search will show you dozens of options, here's one I found in 10 seconds on eBay https://www.ebay.com/itm/403648711177
You'll want to completely take the rear panel off and get some 90% or above rubbing alcohol+Q-tips and completely remove any and all adhesive from the rear glass edges as well as on the edges of the phones frame. Once all the black adhesive is removed from the phone and rear glass you can place the new adhesive onto the frame of the phone carefully, I would check for fitment before peeling the adhesive from it's backing, then place the rear glass on the newly applied adhesive and once done put a few big books on the rear of your phone to apply pressure for a few minutes (about 10) and you're done! You could also use some B-7000 adhesive instead of the pre cut strip but if you want an OEM-like seal and any chance at retaining water resistance, you'll have the best chance with the pre cut stuff. Now obviously do this at your own risk, but I have personally done this to around 4 of my personal phones and have never had an issue with the panel loosening or water ingression but it's all in how well you clean the old adhesive and how well you apply the new stuff. Take your time and you'll be alright.*DEFINITELY INSPECT YOUR BATTERY BEFORE ATTEMPTING THIS* you don't want your phone to catch fire and batteries are cheap to replace.
