SD-RED 60 have lost connect in Standard/split mode

Hello Jirayu Siangsai ,

Thank you for reaching out to the community, In Standard/Split mode, the remote network is managed by the UTM, which serves as a DHCP server and default gateway. Only traffic to specified networks is sent to the UTM. All other traffic is sent directly to the Internet. Where as in Standard/Unified mode, the remote network is managed by the UTM, which serves as the DHCP server and default gateway. All traffic generated by the remote network is sent over the UTM. 

So ensure, if internet connected directly on RED has no fluctuation from the ISP end, you can check the system.log, red.log during the time frame when the issue occurred.

Is it possible this issue happen because SD-RED60 connect to SG firewall?

it may or may not be...