Another mistake
The mistake: "Having older Wi-Fi devices on the network that bring down the new router’s performance".
Many users upgrade their router to newer standards (such as 802.11n or 802.11ac), but fail to realize that older clients accessing the network will bring down the overall performance of the network. That old cell phone or laptop that you use occasionally may be bringing down your performance, because wireless systems act on the “lowest common denominator” principle, setting the router’s performance to the standard with the lowest possible top speed.
The fix: Remove (or upgrade) any older client devices (especially 802.11b) that may still be on the network. You can also configure some routers to operate in an “802.11n only” or “802.11ac only” mode, which will prevent older devices from accessing the faster network.