It could be helpful if You could upload the router's Troubleshoot file. It can be downloaded from the router's WebUI, System → Administration → Troubleshoot page. Just make sure to remove any confidential info (if there is any, e.g., phone numbers, public IPs, etc.) from the router's configuration before uploading the Troubleshoot to a public forum.
However, I'm not likely to see much in the Troubleshoot regarding this issue. But seeing the cellular connection behaviour might provide some insight.
As I've mentioned before, I'm pretty sure this has to do with local conditions. For example, I get ~ 54 Mbps DL speed at my desk, while I get ~ 86 Mbps if I move it near a certain window where the operator's cell is actually visible. If I use a different operator, I get about ~ 34 Mbps. It also varies throughout the day.
I'm not saying that there's definitely no issue, but I am saying that it might prove hard to identify one in this case.