You can go to your local walmart or best buy and get the wifi voice remote, once paired to the tv it control all functions. Roku doesn't sell IR roku tv remotes, that you would need to get from the tv manufacturer. Or go to ebay for used.
Before you attempt to pair do factory reset of the tv, look for reset button at the top of the input panel on the back of tv, so you can get link code and start a new account in your name. So with it power press and hold the reset button or pinhole for over 30 seconds.
The internet isn't needed to pair the voice remote to the roku tv. Have remote ready, with battery door off and one battery installed with the other ready, make sure the tv is on, put the second battery in and press the pairing button in the battery compartment for 3 seconds and release and let it communicate with the tv. It is alright if the light goes out in the battery compartment just let pair.
Please contact your TV's manufacturer directly to report this issue and request additional support. Each Roku TV manufacturer provides direct support and warranty services for their products running the Roku OS.