Update configuration and documentation for 5G modem setup

- Added entries to .gitignore for sensitive/generated files.
- Enhanced troubleshooting documentation in 5G_MODEM_TROUBLESHOOTING.md, including clearer instructions for DHCP issues and AT command responses.
- Improved configure_fm350_5g.sh script with additional logging and configuration options.
- Updated README.md to include new documentation links and a quick start guide for new devices.

5G_MODEM_TROUBLESHOOTING.md

@@ -233,6 +233,12 @@ chown root:dialout /dev/ttyUSB1
 
 **Prevention:** `connect-5g.sh` now checks and fixes a broken AT port automatically before use (recreates the device node if it is a regular file).
 
+### Stray /dev/ttyUSB file (no number)
+
+**Symptoms:** Troubleshoot shows `/dev/ttyUSB` as a regular file (no digit). Real ports are ttyUSB0, ttyUSB1, etc.
+
+**Fix:** Remove the stray file. The connection script now removes it automatically; you can also run: `rm -f /dev/ttyUSB`
+
 ### ttyUSB3 Shows as Regular File (legacy)
 
 **Symptoms:** `ls -la /dev/ttyUSB3` shows `-rw-r--r--` instead of `crw-rw----`

scripts/troubleshoot-5g.sh

@@ -41,8 +41,8 @@ for d in /dev/ttyUSB0 /dev/ttyUSB1 /dev/ttyUSB2 /dev/ttyUSB3 /dev/ttyUSB4 /dev/t
     [ -e "$d" ] || continue
     ls -la "$d" 2>/dev/null
     if [ -f "$d" ] && [ ! -c "$d" ]; then
-        _n="${d#/dev/ttyUSB}"; _n="${_n#/dev/ttyACM}"
-        echo "  -> BAD: $d is a regular file. Fix: rm $d && mknod $d c 188 $_n && chmod 660 $d && chown root:dialout $d"
+        case "$d" in /dev/ttyUSB*) _m=188; _n="${d#/dev/ttyUSB}";; /dev/ttyACM*) _m=166; _n="${d#/dev/ttyACM}";; *) _m=188; _n=0;; esac
+        echo "  -> BAD: $d is a regular file. Fix: rm $d && mknod $d c $_m $_n && chmod 660 $d && chown root:dialout $d"
     fi
 done
 if ! ls /dev/ttyUSB[0-9]* /dev/ttyACM[0-9]* 2>/dev/null | grep -q .; then