Route10 and 2 ISPs

Newbe here… I am a prosumer with a home business and just got Starlink to use as my back up ISP for when my Fiber is down. I configured the Route10 using the default WAN 1 port for fiber, then changed LAN 3 to WAN 2 (changed default WAN 2 to WAN 3) and plugged Starlink into LAN3(WAN2). When I unplug fiber, the route 10 switches my network over to Starlink but it’s supper slow. Starlink right into my network without the Route 10 I get 300 mbps, when I use the configuration above, I get 12 mbps. What am I doing wrong?

Perhaps MTU differences? No idea passed that.. maybe @Alta-Jeff might know as he is very knowledgeable.

Have you looked at the speeds on the Route10 itself? If you give me an invite, I can take a look at a few things. Also, I know that Starlink can sometimes take a few minutes after boot up before the throughput ramps up.