docs/home: Add 5G WWAN modem reference

Notes from bench-testing the Quectel RM500U-EA on tower, bought to
eventually replace stream's Virgin Media WAN. Like the switches and
APs, the module is configured out-of-band and is not referenced by
the flake.

Records what it takes to connect at all: the module must be switched
from its stock NCM composition to MBIM, since under NCM the PDP
context activates but the cdc_ncm link never raises carrier; and the
connect only succeeds with the network-expanded APN read off the
default-attach bearer, as IPv4.

Also flags the two consequences for stream: the SIM is CGNAT so it
cannot carry the public lease stream's WAN currently publishes, and
the bearer reports a /8 that would cover Tailscale's 100.64.0.0/10
if configured literally.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/sites/home/stream.md

@@ -33,6 +33,10 @@ See the consolidated [network assignments](../../networking.md#box-assignments)
 
 ## WAN (Virgin Media DHCP)
 
+A Quectel RM500U-EA 5G modem is being evaluated as a replacement for this WAN; it is bench-tested
+only and nothing here depends on it yet. Note that its SIM is CGNAT, so it cannot carry the public
+lease this section assumes — see [wwan.md](wwan.md).
+
 ### Link and addressing
 
 `wan` is a renamed igc NIC (`00:f0:cb:ee:ca:dd`) towards the cable modem. The modem segment is