NR7302 (ex-Telekom firmware): 0 Mbps Download / 0.5 Mbps Upload with Public IP SIM - Routing issue?

FraTottur

Hello Zyxel Community,

​I am facing a severe routing/NAT issue with a recently purchased used Zyxel NR7302 5G outdoor CPE and I'm hoping some experts here can point me in the right direction.

​The device has a custom carrier firmware (ex-Telekom). I am using it in Italy with an ISP called Alpsim (running on the WindTre network), which provides a Public IP (Denat).

​The Core Issue:

The CPE successfully connects to the mobile network, acquires an IP address, and reports excellent signal metrics. However, the throughput is completely broken: Download speed is hard-stuck at 0.00 Mbps, while Upload works at around 0.50 Mbps.

It behaves as if the outbound traffic is allowed, but all inbound traffic is dropped or fails to route to the LAN.

(To be absolutely clear: the SIM card works perfectly and hits 300+ Mbps if I put it into a 5G smartphone).

​Troubleshooting steps I have already performed:

​Factory Reset via hardware button.

​APN Settings: Configured both the custom APN (myinternet.wind) and the standard ISP APN.

​Strict IPv4: Disabled IPv6/DualStack completely. Set both the APN profile and Cellular WAN 1 to IPv4 Only (as Public IP SIMs here have routing issues if IPv6 is requested).

​Band Locking: Forced the device into "4G Only" to rule out an NSA 5G anchor issue. Still 0 Mbps download.

​Interface Grouping (Telekom specific): I noticed that LAN1 and the WLAN were isolated into separate "Bridge" groups with no WAN assigned. I deleted all these bridges, grouping Cellular WAN 1, LAN1, and WLAN into the "Default" group to ensure they share the same routing interface. The interface updated correctly, but the download remains at 0 Mbps.

​My questions for the community/support:

​Is this a known hardcoded restriction or bug within the Telekom-branded firmware when dealing with Public IP (Denat) SIMs?

​Are there any hidden NAT/Routing menus I need to modify to fix this inbound traffic drop?

​Is it possible to flash the official, unbranded Zyxel firmware onto this NR7302 to remove these carrier restrictions? If so, what is the correct procedure/firmware file?

​I only have a short window to return this device to the seller for a refund, so I need to figure out if it's a bricked carrier-locked unit or if it can be salvaged.

​Thank you so much for your time and expertise!