排查 Ubiquiti 设备的 AT&T 5G 备用网络连接问题

文章背景与核心概要

为了搭建一套移动式“迷你家庭实验室（mini homelab）”，作者测试了 UniFi 5G Backup 作为冗余互联网解决方案。尽管该硬件设计紧凑、非常适合迷你机架，但在将其与 AT&T 商业 5G 套餐进行集成时遇到了重大阻碍。尽管该设备标榜已获得 AT&T 认证，但目前它无法连接到 5G 网络，而是默认降级为 4G LTE，导致网速受限。本文记录了调试过程以及当前 AT&T 网络对 5G RedCap 支持的局限性。

项目与硬件简介

对于我的移动机架项目，我需要一个能够故障转移到固定线路的主 5G 连接。我选择了 UniFi 5G Backup，这是一款支持 PoE 供电的 5G RedCap 设备，最高速率可达 220 Mbps。

UniFi 5G Backup getting 20 Mbps in my studio

For my mobile rack project, I wanted a primary 5G connection that could fail over to a fixed-line connection. I selected the UniFi 5G Backup, a PoE-powered 5G RedCap device capable of 220 Mbps.

尽管 Ubiquiti 明确指出消费者套餐不支持 5G RedCap，但我认为我的 AT&T 企业级“AIA”套餐应该能够正常工作。然而，我遇到了三个主要问题： 1. 固件死循环： 设备在被纳管（adoption）时陷入了更新循环，需要通过手动的 TFTP 恢复过程来解决。 2. 连接性问题： 设备拒绝连接 5G，并回落到了 4G LTE。 3. IMEI 不兼容： 我的 AT&T 客户代表确认，该设备的 IMEI 目前尚不符合其“Internet Air for Business”5G 服务的接入资格。

While Ubiquiti notes that consumer plans don't support 5G RedCap, I assumed my AT&T Enterprise "AIA" plan would work. However, I faced three major issues: 1. Firmware Loops: The device got stuck in an update loop upon adoption, requiring a manual TFTP recovery process. 2. Connectivity: The device refused to connect to 5G, falling back to 4G LTE. 3. IMEI Incompatibility: My AT&T representative confirmed that the device's IMEI is not currently eligible for their "Internet Air for Business" 5G service.

UniFi 5G Backup showing 4G LTE AT&T Connection

调试蜂窝网络连接

为了调查设备为什么无法与 5G 建立握手，我利用 UniFi 网络 UI 的“Debug（调试）”控制台访问了设备的 root shell。

To investigate why the device was failing to handshake with 5G, I utilized the UniFi Network UI's "Debug" console to access the device's root shell.

使用 mca-dump 命令，我检查了调制解调器（modem）的状态：

Using the mca-dump command, I inspected the modem's state:

# mca-dump
{
        "architecture": "armv7l",
        "gateway_ip": "192.168.1.1",
        "hostname": "U5GBackup",
...
        "mbb": {
                "geo_info": {
                        ...
                        "isp": "AT&T Wireless",
                        "organization": "AT&T Enterprises, LLC"
                },
                "imei": "[redacted]",
                "radio": {
                        "5g_sa_mode": false,
                        "band": "eutran-30",
                        "ca_lte": [
                                {
                                        "band": 30,
                                        "dl_bw_mhz": 10.0,
                                        "dl_earfcn": 9820,
                                        "primary": true,
                                        "ul_bw_mhz": 10.0
                                }
                        ],
                        ...
                        "networkoperator": "AT&T",
                        ...
                        "rat": "LTE",
                        "rat_5g_uw": false,
                        "rat_caps": [
                                "5gnr-sa",
                                "lte"
                        ],
                        "rat_mode_active": "LTE",
                        ...
                        "rsrp": -95,
                        "rsrq": -12,
                        "rssi": -68,
                        "signal": 3,
                        "signal_percent": 100,
                        "snr": 13.800000190734863
        "model": "U5G-US",
        "model_display": "U5G-US",
        ...
        "uptime_str": "28m23s",
        "version": "1.4.3.360"
}

输出结果证实 rat_mode_active 设置为 LTE，而 5g_sa_mode 为 false。这表明本地基站要么缺乏 5G SA（独立组网）支持，要么该 SIM 卡在此特定硬件上未被授权使用 5G。

The output confirms rat_mode_active is set to LTE and 5g_sa_mode is false. This indicates that either the local tower lacks 5G SA (Standalone) support, or the SIM is not authorized for 5G on this specific hardware.

结论

将同一张 SIM 卡放入 UniFi 5G Max 中测试，其下载速度接近 600 Mbps，这证明了 SIM 卡和地理位置完全支持高速 5G。

Testing the same SIM in a UniFi 5G Max yielded speeds of nearly 600 Mbps, confirming the SIM and location are capable of high-speed 5G.

UniFi 5G Max getting nearly 600 Mbps downlink

看起来就目前而言，U5G Backup 在 5G RedCap 方面主要针对 T-Mobile 的网络进行了优化。我目前正在等待我在社区论坛帖子上的进一步进展，看看 Ubiquiti 能否解决 AT&T 的认证问题。在此之前，我只能使用 4G LTE 的速度。

It appears that for now, the U5G Backup is primarily optimized for T-Mobile's network regarding 5G RedCap. I am currently awaiting further traction on my community forum post to see if Ubiquiti can resolve the AT&T certification issues. Until then, I am limited to 4G LTE speeds.

关于 TFTP 恢复的注意事项：如果遇到更新循环问题，请确保通过有线以太网连接。TFTP 恢复期间使用 WiFi 连接极易导致数据损坏。

Note on TFTP Recovery: If you encounter update loops, ensure you are connected via a wired Ethernet connection. WiFi connections during TFTP recovery are prone to data corruption.