I connect a different device with a cable and I expect it would get an internet connection or at least a local connection. But, well, nothing happens. There are no messages in dmesg or /var/log/messages on both machines either.

thanks for your response!

1: the cable was indeed bad (it was brand new, so I wasn't suspecting it), but something else is still wrong

2: sorry for just pasting, I'm not really sure how to interpret it client:

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope global lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
2: enp0s25: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether f0:de:f1:5d:57:0c brd ff:ff:ff:ff:ff:ff
    inet6 fe80::29d:f28a:75a7:b5ab/64 scope link noprefixroute 
       valid_lft forever preferred_lft forever
3: wlp3s0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN group default qlen 1000
    link/ether e2:a8:25:b3:ef:1a brd ff:ff:ff:ff:ff:ff permaddr 00:24:d7:a6:ce:e4
4: docker0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN group default 
    link/ether 02:42:0f:11:f5:37 brd ff:ff:ff:ff:ff:ff
    inet 172.17.0.1/16 brd 172.17.255.255 scope global docker0
       valid_lft forever preferred_lft forever

server:

1: lo: <LOOPBACK,MULTICAST,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope global lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
3: enp0s25: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 6e:10:39:1a:0b:03 brd ff:ff:ff:ff:ff:ff
    inet6 fe80::2721:8559:a60d:2e65/64 scope link noprefixroute 
       valid_lft forever preferred_lft forever
4: enp0s29u1u5: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN group default qlen 1000
    link/ether 02:48:11:4d:31:03 brd ff:ff:ff:ff:ff:ff
    inet 192.168.167.220/24 brd 192.168.167.255 scope global dynamic noprefixroute enp0s29u1u5
       valid_lft 3540sec preferred_lft 3540sec
    inet6 fe80::6c7f:73bd:fb6c:bacd/64 scope link noprefixroute 
       valid_lft forever preferred_lft forever

2a: I'm using network manager, it has worked so far. When I try to connect to the wired network manually (normally it does it automatically), nmtui says Could not activate connection: Activation failed: IP configuration could not be reserved (no available address, timeout, etc.). Dnsmasq is running, but I don't know hot to check where is't listening (I tried looking it up, but didn't find anything).

2b: http://ix.io/4z3X/text

3: it only shows the dummy docker thing; when I connect to the wi-fi hotspot in my phone, it does appear and I can ping it

4: I'm not sure, the nftables config is pasted in the original post

«your servers “lan” interface has no IPv4 address»

ok, so how can that be fixed?

«Is this another machine in your network already, or is that a typo?»

Probably a typo, I just copied it mindlessly :/. Now I have this:

dhcp-range=192.168.0.25,192.168.0.50,24h
dhcp-option=option:router,192.168.0.1
dhcp-option=option:ntp-server,192.168.0.1
dhcp-option=option:dns-server,192.168.0.1
dhcp-option=option:netmask,255.255.255.0