The Trick to getting my DGL-4500 Revision A1 to work with 121.NA Firmware.
go into router and reset your router to default settings
then after that reset it again with the button on the back
after that reset update your firmware
then go into router againa nd reset to default settings
then reset it again with the button on the back
dont upload old config, start fresh, has been working so far without wireless drops,
i tested it with 5 laptops all downloading something and watching movies over my NAS storage drive without wireless disconnecting.
might work for you, might not, good luck.