Manual DNS server activation does not work
Hi!
Thanks for the implementation of dnsserver switch!

I tested it on 2 different Fritzbox configuration (7590 and 3490) and it is not working. The behaviour is the same on my both configs: When trying to enable the manual dnsserver via Homekit, the tile remains in state ON a few seconds and then comes back to OFF. Also, after tapping the Homekit tile to activate the dnsserver option, all Fritzbox LEDs start blincking during a few minutes.

Here are the logs after enabling the dnsserver option when tapping Homekit tile:

```
[31.07.2021, 16:02:37] [FritzPlatform] FritzBox 7590 DNS (dnsServer): ON
[31.07.2021, 16:02:37] [FritzPlatform] [DEBUG] FritzBox 7590 DNS (dnsServer): SET CMD: {"xhr":"1","ipv4_use_user_dns":"1","dot_enabled":"0","dot_strict":"1","dot_udp_fallback":"1","dot_fqdn_list":"","apply":"","sid":"c81c89bc6ad938fd","page":"dnsSrv","ipv4_user_firstdns0":"185","ipv4_user_firstdns1":"37","ipv4_user_firstdns2":"37","ipv4_user_firstdns3":"37","ipv4_user_seconddns0":"185","ipv4_user_seconddns1":"37","ipv4_user_seconddns2":"39","ipv4_user_seconddns3":"39"}

```

After a few seconds, the tile comes back to OFF state. And the logs are:

```
[31.07.2021, 16:03:32] [FritzPlatform] [DEBUG] FritzBox 7590 DNS (dnsServer): {"pid":"dnsSrv","hide":{"shareUsb":true,"autoUp":true,"liveTv":true,"faxSet":true,"dectRdio":true,"update":true,"rss":true,"mobile":true,"dectMail":true,"ssoSet":true,"userUp":true,"liveImg":true},"time":[],"data":{"vars":{"ipv6":{"firstdns":{"name":"ipv6_user_firstdns","value":"::"},"userdns":{"value":"0","name":"ipv6_use_user_dns","values":["0","1"]},"seconddns":{"name":"ipv6_user_seconddns","value":"::"}},"dot":{"enabled":false,"strict":true,"fqdns":"","udp_fallback":true},"ipv4":{"firstdns":{"name":"ipv4_user_firstdns","value":"0.0.0.0"},"userdns":{"value":"0","name":"ipv4_use_user_dns","values":["0","1"]},"seconddns":{"name":"ipv4_user_seconddns","value":"0.0.0.0"}}}},"sid":"c81c89bc6ad938fd"}
[31.07.2021, 16:03:32] [FritzPlatform] [DEBUG] FritzBox 7590 Wps (wps): {"NewX_AVM-DE_WPSMode":"stop","NewX_AVM-DE_WPSStatus":"off"}

```

Unfortunately the manual DNS server specified in the config are not applied.


My config is as follow (others options like LED, WPS, Guest activations are working great):
 
```
"extras": {
                "dnsServer": {
                    "active": true,
                    "preferredDns": "185.37.37.37",
                    "alternateDns": "185.37.39.39",
                    "accType": "switch"
                }
```

And I am using plugin version 6.0.14, Homebridge 1.3.4 and FritzOS 7.28.

When setting manually the manual DNS servers addresses in Fritzbox interface and enable them, the dnsserver tile in Homekit changes to state ON (which is clorrect!). Then, tapping on the dnsserver tile in Homekit successfully disables the manual DNS servers in Fritzbox (which is also correct). So the only issue is dnsserver activation.

Thanks in advance for your support and do not hesitate if you need more logs/info.