Go to Settings → My Fire TV → About → Network and make a note of all the settings there (taking a photo with your phone is an easy way to do this).

Go to Settings → Network and select your Wi‑Fi network. Click the remote’s menu button (☰) to forget this network.

Click on the connection again and enter your network password (SSID). Instead of clicking Connect, click Advanced.

Enter the « IP address »…

… and « Gateway » values you noted in step 1.

Enter the primary (DNS 1) and secondary (DNS 2) DNS settings.

DNS list :

// Google
DNS primaire : 8.8.8.8
DNS secondaire : 8.8.4.4

// CloudFlare
DNS primaire : 1.1.1.1
DNS secondaire : 1.0.0.1

// OpenDNS
DNS primaire : 208.67.222.222
DNS secondaire : 208.67.220.220

// Verisign
DNS primaire : 64.6.64.6
DNS secondaire : 64.6.65.6

// Quad9
DNS primaire : 9.9.9.9
DNS secondaire : 149.112.112.112