If you have access to another wifi net, like maybe a mobile phone hotspot, you could set up using that. Once set up, Settings > System > About will give you the MAC address. Then you can set up with your router and reset your Roku's network settings for its new net.
- Roku Streaming Stick +, 3810X, Ser YH0059427035, wifi - Samsung UN55ES6100
- Roku Streaming Stick, 3600X, Ser 5S56D8240827, wifi - JVC EM37T
- TCL Roku TV 43S425/C107X, Ser X000001R60KV, wifi