Unsatified1
I would not Factory Reset the Roku device yet until I had it connected to internet. Makes it a lot harder to diagnose and narrow down issue and you may not be able to get past the setup screen.
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If you have one of the older Roku Express + devices (models 3710, 3910, or 3931), then you have a 2.4ghz-band only device. (You can verify your model number from Settings/Systems/About). This means you can only connect to the 2.4 ghz band of your network.
Notwithstanding your ISP telling you there is nothing wrong on their end, there is a known issue with cable ISP routers (Xfinity/Comcast, Cox, and AT&T) having disconnection issues with 2.4 ghz devices. The solution has been to login to your router or use their accompanying app and change/edit the 2.4 ghz wireless settings. Essentially first step is to change Wireless Mode from g/n to b/g/n.
The above will likely fix your disconnection and troublesome reconnection issues. In some cases, you may have to have the ISP provider make these changes for you if you cannot access your router settings. This really is a 5 minute fix so worth a shot. Below are the links on how to make these changes to your router's setting depending on your ISP.
Xfinity / Comcast → ( https://www.xfinity.com/support/articles/change-wifi-mode-admin-tool-xfinity-xfi ) with a walkthrough ( https://community.roku.com/t5/Network-Wireless-Wired-Connections/Error-014-30-stuck-for-three-days/m-p/749057#M21380)
Cox → (https://www.cox.com/residential/support/managing-your-in-home-network-with-my-wifi.html)
AT&T → (https://www.att.com/support/article/u-verse-high-speed-internet/KM1046172). You can select your appropriate router from the drop down menu.
**Restart both your Router and your Roku device after making the above changes.
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We have to eliminate this known issue (switching wireless mode to b/g/n) before making any other changes to your settings. ]
Feel free to post back with an update or if you need more help. Please include the info requested previously re: Roku model number, router model number, and ISP provider. There are sometimes unique resolutions/issue to certain devices and can help narrow down issue further with more specifics. Also include any error messages you are seeing on screen. (normally there is a number attached to the error).