Permissions are a mystery to most windows users.. it is there but well hidden.
Ownership of a folder and files in the folder is difficult to control .. even Apple are trying to hide how permissions affect things.
Firstly if you want a folder to be shared with windows.. create it in windows. Then when you copy the files to the folder from the Mac it should inherit the permissions from the folder and windows can read it..
But if that doesn't work there is a dirty trick..
I presume you have installed airport utility in your windows machine. (I am using the Mac version so slightly different font but matches pretty well.. functionality exactly the same).
Go to Disks/File Sharing.
Turn on Guest access to Read and Write if you want to do both.
When you do this.. all permissions on the TC will change for all users.. windows or Mac.. so everyone has access.
So don't do it if you want to maintain security.. but for most people sharing the TC in a local network it is easiest way by far to get things working.
I have checked this works on latest version TC even though windows version is below spec to configure it.. and got another user here to double check with High Sierra.. and this indeed allows files that even High Sierra cannot access to work.