High level network support need PLEASE!!

My Windows 11 laptop has trouble accessing banking and other secure websites when connected to my Xfinity XB8 gateway's main Wi-Fi network, [Edited: "Personal Information”] 5G. The laptop uses a Realtek RTL8821CE Wi-Fi adapter and connects on 5 GHz with a strong signal.

The laptop previously received self-assigned IPv4 addresses (169.254.x.x) with no IPv4 gateway. Windows Event Viewer also recorded TCP/IP Event 4199, reporting an IP address conflict for 10.0.0.99 with MAC address [Edited: "Personal Information”].

The laptop now receives 10.0.0.242 with gateway 10.0.0.1, but USAA still fails with ERR_ADDRESS_UNREACHABLE. DNS lookups to Xfinity DNS server 75.75.75.75 time out on the main network but succeed on my TP-Link RE105 extender network, [Edited: "Personal Information” 5G_EXT. Banking websites also work on the extender.

The gateway was recently replaced, and changing the Wi-Fi security setting from WPA3 to WPA2 did not resolve the issue. Please investigate the XB8's DHCP/IP conflict handling and DNS/connectivity for clients on the main Wi-Fi network. I need to keep the extender working for my appliances, so please avoid splitting the Wi-Fi bands or resetting the network without discussing the consequences first.