I stumbled upon the following observation. I have a router with an dynamic external IP with changes every 24h. It can be reached via it's dyndns test.test.zzz. Client1 and 2 have the same config to reach the endpoint:[Peer]
PublicKey = 5JZzs90SBYzXc8pfDW......
Endpoint = test.test.zzz:443
PersistentKeepAlive=3600
Client2 Ext IP 145.x.y.z wg0 10.2.1.3
    |
    |
Router IP 10.1.1.1  Dynamic Ext IP 83.x1.y1.z1 (after 24h/change to 84.x2.y2.z2) dyndns: test.test.zzz
   |                                                  |
   |                                                  |        Client1 10.1.1.3 wg0 10.2.1.2         Server 10.1.1.2 wg0 10.2.1.1
After the external IP address changes I cannot reach client1 from server or client2 on it's wireguard wg0 device.
wg show on client1 
peer: 5JZzs90SBYzXc8pfDW......
endpoint: 83.x.y.z:443
allowed ips: 10.2.0.0/16
latest handshake: 8 hours, 14 minutes, 42 seconds ago
transfer: 9.98 KiB received, 49.73 KiB sent
persistent keepalive: every 6 minutes

wg show on server
peer: 9BhasduiUTa3d.....
endpoint: 83.x.y.z:43765
allowed ips: 10.2.1.2/32
latest handshake: 8 hours, 14 minutes, 42 seconds ago
transfer: 22.73 KiB  received, 169.98 KiB sent

Both devices still list the old external IP. Although on client1 and server test.test.zzz resolves to it's new external IP 84.x2.y2.z2. I know that I could fix this by entering the internal IP of the server as the endpoint address on client1, but I thought that if the server couldn't be reached any more a new handshake process (with DNS lookup) would be initiated?