Under closer examination onsite, we found that there was a data switch between the Sonicwall router and the ISP's router. This data switch works ok with Auto Negotiate. When changing the speed to 100Mpbs Full duplex on the Sonicwall, the switch would revert to half duplex. I also see that the connection to the ISP router is also connected at half-duplex.

The switch is an unmanaged switch, so we are going to see if the carrier can switch to Auto-negotiate to match the data switch, or we will need to replace that switch with one that we can set to 100Mbvps full.

I don't think that removing this switch is an option because there appears to be 2 other routers connected.