Garebear68
What is the model number of your older Roku device. (Settings/System/About).
What network band are you using for both your Roku devices. (Settings/Network/About).
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If your Express 4k+ keeps disconnecting, try connecting it to the 2.4ghz band and see if you have a more stable connection. It is possible that your 5ghz band is currently configured to use DFS channels (52-140) which Roku cannot see or use. If the router switches to a DFS channel, your Roku will lose connection. If you are having to restart your network equipment to regain connection, this would be the first setting I would look at. Either disable use of DFS on the 5ghz band, or manually assign a wireless channel of 36-48 or 149-161 and see if performance stabilizes.
While you are at it, disable the Power Saving feature of the Express 4k+. (Settings/System/Power/Auto Power Savings/20-min/[uncheck to Disable] ). This can cause some stubborn reconnections. If you are currently using the TV USB power port for power, instead try using the wall adapter and plug directly into wall outlet. (any cell phone charger adapter 5volt, 1amp will do). An under-powered device will behave unpredictably as well.
The above should allow to rule out some possible culprits.