Hi @momopoco ,
I doubt that you will be able to reseal it back to IP68 as well as the factory did but you can only try.
First check that the frame and back cover are not bent, preventing a perfect seal.
You may have to remove the back cover and remove the old adhesive and clean the surfaces of the frame and cover using Isopropyl Alcohol 99%+ (available from electronics parts stores) and then apply new pre cut (select option from tape size search box) TESA adhesive strips and reseal.
I suggest that you disconnect the battery from the systemboard (just disconnect it, do not remove the battery) whilst cleaning etc to avoid any possible electrical problems from occurring. Even IPA will conduct electricity when it is still wet, although it leaves no conductive residues after it has evaporated. There is always power present at some points on the systemboard even when the phone is off. The power button is not a power isolating button. Its function is to signal the intentions of the user to the OS e.g. turn on turn off etc. Think of the phone as being in an extremely low power state when off rather than being disconnected from any power source.
Mechanical problems e.g. tools slipping, physical damage etc are a different matter so be careful. Don’t forget to reconnect the battery before sealing the cover back on. ;-)
Here are some links that may help:
Samsung Galaxy Note20 Ultra Battery Replacement