Maybe get the same type router as your Grandma's and you will be set, if it is not cost prohibitive or continue to tinker with your router and Diamond and all of a sudden you will see you are connected
Hard to troubleshoot without being there as there could be multiple variables. The fact that your Diamond was working at another location should help you to troubleshoot backwards, so to speak.Also at the other connection, was there a data connection over the air from your cellular carrier in addition to your Grandma 's WiFi or strictly WiFi only?