150 and 300Mbps are theoretical maximums for 1 and 2 antennas on N, respectively, assuming a 40MHz wide channel on 2.4GHz, which really isn't possible unless you live on a farm with no nearby neighbors.
At 20MHz and one antenna (unless your cameras have two) the theoretical maximum is 72.2Mbps before overhead and after it you would be doing well to get an actual 54Mbps transfer rate for data. Note that some wifi chips like Atheros do not allow you to set a short guard interval on 20MHz 2.4GHz so the theoretical maximum is even lower at 65Mbps.
So why are you getting 10 or 2 instead? Because older drivers and firmware aren't as good with coping with interference. There are lots of things like security cameras and baby monitors nowadays using wifi frequencies but which are not wifi so won't appear in any wifi traffic analyzer tool. They broadcast spread-spectrum so corrupt wifi packets to require retransmits which slows things way down.
I would suggest loading FreshTomato firmware onto that router because Broadcom MIPsel routers continued to be made long after that one, so much newer drivers are available in that and the latest was just released this month. While DD-WRT is also listed as supporting the E900 I would avoid it simply because the latest one anyone has reported as working in their forums is from 2020.
BTW most cable internet is faster than the rated speed whenever the excess bandwidth is available (with Comcast it's typically 118% so for 100Mbit service you usually don't want to set QoS much lower than 118Mbps) however your old router has only 10/100 ethernet ports so you'll obviously never see this, even wired.