Hello,
You did not mention the type of LTE modem what you use. But:
your 800$ or more iPhone has a great LTE modem, probably CAT20 that supports different types of aggregation. I bet your LTE modem in the router is much simpler therefore if in this case it can use only one band 7 which is probably 20Mhz wide then 18Mbps is quite good speed which can go up or down depending on the network congestion (number of users of this particular cell tower).

Another possibility (but it is less likely) - traffic shaping i.e. your carrier limits your speed and it can be done due to a number of reasons. But having 18Mpbs I guess is a good thing i.e. nothing to complain about.
I have 2 LTE modems (Netgear, Sierra Wireless), my Samsung s10 (LTE Modem CAT20) gets up
200 Mbps down and 100 up, SW (CAT6) - up to 50-60 down / 30-40up, Netgear (no aggregation) 20 down -10/25 Mpbs up with the same SIM card.

Last but not least: phones have always highest priority on the network comparing to tethering phones, modems etc.