Try connecting to Hypixel or any other big server. If that doesn't work, it's your internet.
If it does, go to your Minecraft launcher and make sure and double check that it's 1.12.2. To be extra sure, use OptiFine.
If it still doesn't work, try deleting Dogcraft from your server list and use direct connect instead. Make sure it is actually server.dogcraft.net and not something else. If it works, try adding the server again, if it doesn't, then I don't think I can help you there.