Low Internet Speeds with Mesh Network

I have a newer Xfinity Gateway (modem/router) connected via ethernet to a Google Nest mesh network. The past 4+ years this has worked great, but recently my internet speeds have dropped significantly (less than 100Mbs) from my mesh network to my devices even though my internet speed from my Xfinity Gateway to the mesh network is still good (over 800Mbs). I've tried disconnecting/unplugging both the Xfinity Gateway and the Google Nest devices and restarting with no success. I've also disconnected my Google Nest mesh network and tried to re-setup up my Xfinity Gateway through the app "Activate WiFi and more" section with no success. I would like to troubleshoot further, but I no longer have access to the Admin Tool through http://10.0.0.1. I also cannot "Allow Admin Tool access" through the Xfinity app because my app screen doesn't have the buttons (photo attached)

for "View WiFi Equipment" or "Advanced Settings". The Xfinity Assistant has been less that helpful and I've been unable to connect to a live agent so far.