ESP8266 triggers ICMP Flood DoS on router (was New router from ISP and now no ESP8266 modules will connect)

Code that has been working for ages now suddenly will not connect to the new router that I have been sent by my ISP.

I have tried every combination of encryption and key exchange on the router, but nothing makes a difference.

Here's a full debug log:

```
[WIFI] delete old wifi config...
scandone
[WIFI] start scan
:ur 1
scandone
[WIFI] scan done
[WIFI] 5 networks found
---> 0: [2][xx:CF:xx:C2:xx:2C] MySSID (-56) *
1: [6][xx:FE:xx:4D:xx:B9] Neighbour-1 (-86) *
2: [6][xx:3B:xx:66:xx] Neighbour-2 (-86)
3: [6][xx:FE:xx:4D:xx:BA] Neighbour-3 (-86)
4: [11][xx:10:xx:0C:xx:BD] Neighbour-4 (-88)

[WIFI] Connecting BSSID: xx:CF:xx:C2:xx:2C SSID: MySSID Channel: 2 (-56)
scandone
state: 0 -> 2 (b0)
state: 2 -> 3 (0)
state: 3 -> 5 (10)
add 0
aid 2
cnt

connected with MySSID, channel 2
dhcp client start...
wifi evt: 0
ap_loss
state: 5 -> 0 (0)
rm 0
reconnect
wifi evt: 1
STA disconnect: 8
scandone
state: 0 -> 2 (b0)
state: 2 -> 3 (0)
state: 3 -> 5 (10)
add 0
aid 2
cnt
ap_loss
state: 5 -> 0 (0)
rm 0
reconnect
wifi evt: 1
STA disconnect: 8
scandone
no MySSID found, reconnect after 1s
wifi evt: 1
STA disconnect: 201
reconnect
scandone
state: 0 -> 2 (b0)
state: 2 -> 3 (0)
state: 3 -> 5 (10)
add 0
aid 1
cnt

connected with MySSID, channel 2
dhcp client start...
wifi evt: 0
ap_loss
state: 5 -> 0 (0)
rm 0
reconnect
wifi evt: 1
STA disconnect: 8
scandone
state: 0 -> 2 (b0)
state: 2 -> 3 (0)
state: 3 -> 5 (10)
add 0
aid 2
cnt

connected with MySSID, channel 2
dhcp client start...
wifi evt: 0
ap_loss
state: 5 -> 0 (0)
rm 0
reconnect
wifi evt: 1
STA disconnect: 8
scandone
state: 0 -> 2 (b0)
state: 2 -> 3 (0)
state: 3 -> 5 (10)
add 0
aid 1
cnt
ap_loss
state: 5 -> 0 (0)
rm 0
reconnect
wifi evt: 1
STA disconnect: 8
[WIFI] Connecting Failed (6).
:ur 1
:ur 1
:ur 1
:ur 1
:ur 1
:ur 1
```

Now, I have no idea what any of that means, but from my reading it looks like the AP vanishes part way through connecting. But if someone could help decode that lot it would certainly help me work out what is going wrong here...