Small waterproofing ruber piece fron the USB Port

Hello, there is a small rubber part that fallen from my ZU's USB port. I believe it is a waterproofing part.
Has it happened to anyone else? How can I put it back? I've tried to insert in back but its doesn't stays in place no matter how I put it, was it originally glued?

Need a new battery but gotta retain waterproofing

Hi, is there a DIY method or place (EU) to change the battery and maybe the backcover, which can guarantee the waterproofing will still be ok after the procedure?