Mobile Data and APN

Mobile Data and APN

In one sentence: put a SIM with a data plan in the device and Linux connects to 5G (NSA) or LTE by itself and shares the connection with everything on its Wi-Fi and USB.

The simple version: the modem is a little phone inside the box. Linux dials in for you, keeps watching the line, and dials again if it drops. Most SIM cards need no settings at all.

flowchart LR
    Phone[Phone / laptop] -->|Wi-Fi hotspot| Dev[F50 / U30 Air<br/>running Linux]
    PC[Computer] -->|USB network| Dev
    Dev -->|5G NSA / LTE modem| Cell((Mobile network)) --> Net[Internet]
    Dev -.->|or: Wi-Fi client of another network| Router[Other router] --> Net

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Check it

sudo mobile-data status

No sudo on OpenWrt. The connection reconnects by itself after modem resets (mu300-mobile-data-watch), unless you took it down yourself with mobile-data down. Other subcommands: up [APN], down, signal, sim-reset.

No internet? First check that the SIM has a data plan. A missing plan looks like a connection that keeps dropping.

The APN

Leave it empty unless your carrier says otherwise. Empty means: use the data context the SIM itself defines, which is what most carriers expect.

System	Where
Ubuntu	/etc/mu300/mobile-data.conf: MU300_APN and MU300_PDP_TYPE (an example is in /etc/mu300/mobile-data.conf.example)
OpenWrt	LuCI -> Network -> Interfaces -> wan, or uci set network.wan.apn='…'; uci commit network; ifup wan

An APN with a user name and a password (a private APN, some MVNOs): set them next to the APN. They need an APN; printable ASCII, at most 64 characters each, no quotes or backslashes. The authentication is pap, chap, both (the network picks, and the default) or none.

System	Where
Ubuntu	MU300_APN_USER, MU300_APN_PASSWORD, MU300_APN_AUTH in /etc/mu300/mobile-data.conf
OpenWrt	the same page (APN user name, APN password, APN authentication), or uci set network.wan.username='…'; uci set network.wan.password='…'; uci set network.wan.auth='chap'; uci commit network; ifup wan

The file that holds the password must be readable by root alone; the dial makes it so (chmod 600) and does not start otherwise, and a user name or password without an APN is refused rather than left out. This is new and has not yet been tried against a network that checks the credentials; if your carrier needs it, a report either way is welcome.

Data usage

The device counts its mobile data per day and per billing cycle, both ways, the way the operator counts it. The count survives reboots and switching between systems, and nothing is counted twice. On OpenWrt with the control panel this is Cellular > Data usage.

mu300-traffic                       # today, this billing cycle and the cap
sudo mu300-traffic set reset-day 1  # the day a billing cycle starts
sudo mu300-traffic set cap 50G      # a monthly cap; it warns at 90 % unless you set another level
sudo mu300-traffic set cut on       # optional: mobile data off at the cap, back on when the next cycle starts
mu300-traffic days                  # the last days with traffic

Network, band and cell locks (control panel)

On OpenWrt with the control panel, Cellular -> Network locks sets network mode, band, cell and EN-DC locks that persist across reboots and are replayed at boot. The locks are stored in the modem's own memory, so they also apply in Android. "Reset all to automatic" on that page (or unisoc-modem lock reset on the command line) clears them. The Status dashboard shows live signal, bands, cells and temperatures. See Control Panel (LuCI).

USSD (balance, own number)

sudo mu300-ussd '*101#'

The code depends on your carrier.

TTL

sudo mu300-ttl set 64     # a fixed TTL for everything leaving through mobile data
sudo mu300-ttl off        # back to the default

Also in mu300-toolkit -> Network -> TTL, or the panel's Cellular -> TTL. On the mainline kernels the rule is in tc and flow offloading stays on; on 5.4 it is in nftables and offloading is off while a TTL is set.

Changing what your device sends (TTL, band locks, VPNs) may break your operator's terms or local law. Check before you turn something on.

IPv6

Plain OpenWrt uses prefix extension; OpenWrt with the control panel relays IPv6 from the carrier (router advertisements and NAT66) instead. Since v2026.10.18, when the carrier moves the device to a new /64, the old prefix is withdrawn, so clients stop using it without a restart of the WAN.

Wi-Fi beats mobile data

When the device is also joined to another Wi-Fi network as a client, the Wi-Fi route (metric 50) wins over mobile data (metric 100), so the SIM stays idle. See Wi-Fi Hotspot and Client.

"Websites think I am in another country"

The device has no GPS, so websites guess your location from the IP address, and mobile operators often look like a different city.

Saving power (U30 Air)

mu300-power can switch the radio to LTE only or off while nobody is connected. See the README's power row and Hardware Notes.

Deep details (radio registration, the data bearer, modem resets): FINDINGS 15, 26.