Is there any way to avoid triple NAT in his scenario.

First is the T-mobile can that we all know has virtually no configuration options. 

Next is a Sonicwall TZ270 firewall. Next is the Google Nest WiFi mesh system. 

So it currently goes like this:

T-mobile Lan (192.168.12.1) > Sonicwall WAN X1 (192.168.12.231) 
                                                Sonicwall LAN X0 (192.168.168.168) > Google WAN (192.168.168.65)
                                                            Google LAN (192.168.86.1) > Switch (192.168.86.x) > LAN devices

Any way to go from triple NAT to double NAT?