Have you tried resetting the network settings on the problem device?
Hi - what is the modem model, does it broadcast separate SSIDs for 2.4 Ghz and 5 Ghz bands and what is the model of the Tablet - is it only capable of 802.11 b/g/n 2.4 Ghz
I tried reserving an ip for the mac address, but that did not work. the modem is a sagemcom 5366
You Nexus may be remembering old details. Have you tried going to the Nexus network settings and telling it to forget the current connections for your 5366? i.e. you need to get to the point where the Nexus ask you the enter the password for the 5366.