I purchased a RUT955 in August 2021.
It worked fine initially with RUT9XX_R_00.06.07.7, the firmware the router came with. I was able to change the SSID and password.
I then upgraded to RUT9_R_00.07.00. As I had not kept the settings, I wanted to change the SSID again to the same values I had used before. At the same time I added a second (guest) wifi network, and set a data limit. This however proved to be a huge mistake. Following was a neverending series of problems that I am unable to solve.
After I had changed the SSID, the router declined any attempt to connect (LAN / WiFi) any device. I only gained access again, by resetting to factory settings. OK, after several failed attempts, I left the standard entries, that were shown after each reset. At least, I was able to connect my devices and open the WebUI with these entries.
The data limit settings are unclear or not working. The field you can enter a value shows no unit. I set 5000 assuming this would allow 5000 MB = 5 GB to be transferred. However, little or, more likely, no data at all were transfered.
The WebUI and the lights on the router show that the SIM (Telekom Germany) is online with strong signal, but I do not get access to the internet. I tried different devices (iPhone, iPad, Apple TV 4K, laptop with Windows 10), connected via cable and wifi. None of them worked.
I then connected the router, installed in a motor home, to our wifi network at home. All of a sudden, all devices connected to the router had access to the internet. As soon as I turned the external WiFi off, connection to the internet was lost.
There were a few cases that gave me internet access via SIM after factory resets, but never for long. Once the connection was lost, it never came up again (except by one of several new full resets).
Reading the forum, I learnt that you suggest to re-install the firmware. I first tried RUT9_R_00.07.00. No improvement. I tried a second time. Again, no changes.
I then downgraded to RUT9XX_R_00.06.07.7, as the router had worked fine with this firmware. However, the only changes I could see were the look and feel of the WebUI, all other problems remained the same.
I upgraded back to RUT9_R_00.07.00, but again, no improvement.
I hope you are able to provide a solution that works. Otherwise, I am considering to return the product. As it is only 6 weeks old it is still under warranty.