There are no issues with this router and 98SE - I sort out my friends PC`s with 98SE by bringing their computer to my house, setting up a wifi connection to this router and then updating their system from the internet. Some thoughts then:
Is it a range problem? What is the signal strength from the summer house? Have you enabled encryption and if so did you save the configuration as a profile in the D-Link USB adapter user interface so they can activate the connection?
In my experience the weather (atmospherics) affects the signal strengths greatly at a distance. This can drop from 90% to 60% on a day to day basis with a computer and router in the same positions. Sometimes simply switching off this router for ten minutes and then rebooting the computer actually improves the connection but don`t ask me to explain why!