Your contract is with the people you purchased the Disgo from and you should really be talking to them about the guarantee. It may have a design fault that you are unaware of did you perhaps drop it on the buttons? If it is a known design fault (have you Googled to see if this is a common problem?) then the damage will be covered by the guarantee but if it is accidental damage you may not be covered. Talk to the supplier about the guarantee terms.
OnePlus 5T review: Hands-on