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/README.md

@@ -59,7 +59,9 @@ colony (physical VM host, ams1)
 
 Redundant routers, VM host, storage, IoT containers and the workstation — see
 [`sites/home/README.md`](sites/home/README.md). The hand-configured switch fabric (jim/dave/brian)
-and the Digiweb WAN path are documented in [`sites/home/switches.md`](sites/home/switches.md).
+and the Digiweb WAN path are documented in [`sites/home/switches.md`](sites/home/switches.md); the
+5G modem being evaluated as a replacement for `stream`'s WAN is in
+[`sites/home/wwan.md`](sites/home/wwan.md).
 
 ```
 h.nul.ie