Compatibility XB8 - Eero pro 6E

Hi,  I have the XB8. I bought and eero pro 6e mesh. When I turn on bridge mode the system doesn't work. If I disable the bridge mode they connect but the performance is really poor. With an Ethernet directly to the modem almost 1000bps but with the eero connected with a cable to the xb8 is extremally poor around 120mbps. I think that the cause is that there are 2 different signals. I worked for hours with eero tech support and they run several tests and configurations without success. They sent me a new one but i have the same problem. They suggested to contact Xfinity to find out if you think that there is compatibility problem.  do you have any suggestion/guidance? Thank you very much!