MR5200 unable to connect after prolonged time

I’ve recently bought an M5 and have generally been very happy with it, but I’ve noticed if I leave it on for an extended period of time (say longer than 24 hours) already connected devices are OK but other devices can’t reconnect to it, until it restarts. It’s generally only used by 6-8 devices at most so it doesn’t seem like it should be anything simple like the dhcp pool being fully allocated (the default pool allowed for 80 addresses)