The suggestion to restart the router is not made lightly. It's not guaranteed, but very often a router restart restores a Roku's network connection, even when other devices have no trouble connecting. In little more time than you spent typing your two objections to the suggestion you could have found out if it worked for you. But you may not have this option if it is a system maintained by your apartment management.
Next best suggestion may be to clear out your connection info and try to establish the connection all over again. Settings > Advanced system settings > Network connection reset > Reset connection.
OK, the Express is 2.4 GHz only, so the Mode setting suggestion I made above is appropriate. I am assuming you can see your wireless, but just not be able to connect to it. Again, try a hotspot to see if you can connect and get your OS updated.
No problem. 🙂 Virtually all smartphones offer the ability allow other devices to connect and share its Internet connection. Basically, it just appears as another available wireless connection to your Roku. I can't speak to Android based phones, but on the iPhone it's under Settings/Personal Hotspot. Under that menu, you can see (and change) the name that is broadcast (the SSID) and the password. Then you simply turn it on and then search for a new connection on your Roku and connect to it.
That said, there's always the chance that your phone's data plan doesn't offer hotspot functionality. If that's the case, see if you can use someone else's temporarily so you can see if there's an OS update that might resolve the issue. It wouldn't be the first time that a Roku could not connect to a home network, and after using a hotspot to update the device it could again connect to the home.
Speed shouldn't matter, as long as you can connect and see if Roku has released an OS update to hopefully resolve your connection issue.