Such big variances in speed could be explained by a few factors:
- Weather conditions;
- Poor signal;
- Cell tower load;
- Changes on the carrier end (power-saving, etc.);
If the last two factors is influencing the speeds the most, then there is not much we can do, but what I suspect is happening, is when the router is connected to a higher throughput band (e.g. B3), it can achieve those higher speeds, but during bad weather, or generally decreased signal, the RUT950 drops down to a slower band, that provides greater coverage.
I would recommend checking what band you are connected to when the speeds are high by navigating to Status → Network → Mobile. When the speeds get low, check if you are still connected to the same band. If you are not, you can lock to a previously used band. Instructions on how to do so and some more additional information can be found in this Wiki article.