Do you remember the SSID (network name) and password of the last network the TV was connected to? If so, it may be possible to get your TV connected to the new network using your phone's hotspot configured as the old network and another device with the Roku app connected to the same hotspot to act as a remote. If not, I think you need an IR remote unless you want to factory reset the TV. Note that if you do factory reset the TV, the other trick using a phone hotspot is no longer available.
If you still have the old router, I would think you could accomplish the same thing as the hotspot trick by having the new and old router powered up at the same time and the phone connected to the old router. I've never tried it, but it seems like it should work. I don't think the router would even need to be connected to the Internet, but it also seems more and more that Roku devices don't function without an Internet connection.
How to set up your Roku® voice remote | Official Roku Support
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