A few things that you need to look (in this order - easy to hard) :

check the spacing of the rear, if it is less that 130mm you cannot have an MTB 7 freewheel there and it is probably the frame that is the problem and not the RD

if that is the case, manually (i.e. don't use derailleurs, but with the derailleurs on - and wear gloves btw) put the chain on the biggest cog up front and on the biggest cog in the back. If there is not enough room to do this, the chain is the limiting factor and you need a different chain. (and, yes, this is not 'textbook' because you are running cross gears at this point, but the chain should be able to do that)

If you can do that manually, you have to adjust your RD in a way to be able to do it physically...

Unless you are running something in the 30s this XT RD should be able to handle at least 28 teeth...