Both mine are secured with large sticky pads
I put personalised plates on, the previous plates were affixed the same way.
I removed the old pads by 'sawing' with fishing line.
The residue was then removed with WD40
I purchased a large pad that covers the whole plate from https://www.tape2go.com/guru-auto-mount-number-plate-tape-7127-p.asp An excellent product that securely fixes your plate.