You beat me to it. If OP has lost or given away the AC adapter, he (she?) can use most any cell phone charger that has a detachable cable and a standardized USB port.
Aside from that, if a router reboot allows the Roku device to reconnect, then it's probably not the Roku device that is "garbage," lol, because it's obviously ready to make the connection. Try accessing your router's configuration web page and, in it, assigning a static IP address to the Roku device (et al). While you're doing that, ensure that you or someone else hasn't set a low limit for the number of allowable devices/connections. If this device is only capable of 2.4 gHz b/g/n connections, have your router prioritize such things (if it is capable of doing so). Also consider changing your router's password. If this appears to make this issue go away for a short time, but then it suddenly reappears, suspect local "script-kiddies" (who may or may not actually be kids) stealing access to your WiFi connection. I don't know what you could do about that; the security of consumer-level electronics is roughly on par with a screen door that uses a hook & eye "lock." That is to say, they do not really have any. People can hack WiFi networks in 10 minutes or less using freely available - and free - software. If the wireless router has WPS (WiFi "Protected" Setup) - allowing you to connect devices that also have it without needing to enter your password - and you've not disabled the function, then there's a fair probability that a person could hack it in less time than it would take you to enter a reasonably long password. The solution used to be simple - disable WiFi and connect everything via Ethernet. But that is no longer viable, because many devices these days do not have a physical connection point or, in the case of cell phones, require an adapter.
OP: Again, the Roku device is not the problem. It requires a connection to the Internet via WiFi for even basic functionality and, therefore, begins attempting to establish one as soon as it is powered up. You have proven that is working fine by rebooting your router and observing that the Roku device is then able to make that connection. Your problem lies elsewhere, bud. Your wireless router wouldn't happen to be your cable company's Internet access device, by chance, would it? Because some of those seem to be of even lower quality than the average $20 piece of Chinese dumpster fodder that gets sold on Amazon (which is really saying a lot).
