Setting up a DHCP subnet running on several interfaces

So my problem is as follows:

I want to run a DHCP server on my Ubuntu 18.04 LTS. So I installed ics-dhcp-server. Then I changed the configuration files according to this guide. When connecting one single client (dhcp-capable device) it actually worked and my settings look like this:

in /etc/default/isc-dhcp-server

#INTERFACESv4="enp2s0 enps3s0 .... and so on, for all of my 7 physical interfaces"
#INTERFACESv6=""

in /etc/dhcp/dhcpd.conf

option domain name "localhost";
default-lease-time 600;
max-lease-time 7200;
ddns-update-style none;

authoritative;

subnet 10.5.5.0 netmask 255.255.255.0 {
  range 10.5.5.100 10.5.5.200;
  option domain-name-servers 10.5.5.99
  option subnet-mask 255.255.255.0
  option routers 10.5.5.1
  option broadcast-address 10.5.5.31
  default-lease-time 600;
  max-lease-time 7200;
}

Of course I enabled and restarted the service in systemctl.

My interfaces in network manager are configured on a static IP (10.5.5.2 and ascending, have a netmask of 255.255.255.0 and as gateway they use 10.5.5.1).

Now the problem starts when I want to connect multiple devices to any of those physical interfaces. Even though they are all listed when calling dhcp-lease-list, I can only connect / ping to one randomly chosen device. It also only works with pinging the IP directly so hostname propagation also seems to be broken. Bonus points for anybody who solves this as well...