Grandstream 2000 losing registration every night

We are using a hosted Asterisk instance at and have 4 user extensions in our office behind a DSL modem/router. All works fine during the day but each night the phones lose their registration and have to be hard reset in the morning. We are using Grandstream GXP 2000. All firmware and software is up to date.

We have tried several different NAT traversal settings on the GXP phones as well as different register expiration settings

Any suggestions?