The reason for low download could be simply high load or limited bandwidth of the cell tower your router is connected to.
Carrier aggregation is performed by the software within router's modem. I would suggest to test, what speeds are received, when the same SIM is used inside a smartphone.
A couple of things to tinker around with, could be setting different frequency bands. These can be set manually in Network -> Mobile -> General section.
You could also enable Software flow offloading in Network -> Firewall -> General settings section.
Is your SIM configured to receive APN automatically, or have you preconfigured custom value? Could you check both settings?
Lastly, modem firmware update could be done.