I use Ubuntu 18 with KDE Plasma, and I cannot use 2 network interfaces at same time. I have "Wired connection 1" from my IPS (ethernet cable) and "Wired connection 2" using my phone "USB tethering", always works only one interface, if I use phone's connection it works, and if I use connection from my ISP it works too, but if are both connected, only one works, another is like 0 KB/s, I tried several speed tests.
This is from ip addr
:
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 host lo
valid_lft forever preferred_lft forever
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: enp3s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
link/ether d8:50:e6:54:28:d1 brd ff:ff:ff:ff:ff:ff
inet 192.168.1.25/24 brd 192.168.1.255 scope global dynamic noprefixroute enp3s0
valid_lft 599667sec preferred_lft 599667sec
inet6 fe80::6bd6:81a4:afcf:564f/64 scope link noprefixroute
valid_lft forever preferred_lft forever
3: virbr0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
link/ether 52:54:00:3c:0c:63 brd ff:ff:ff:ff:ff:ff
inet 192.168.122.1/24 brd 192.168.122.255 scope global virbr0
valid_lft forever preferred_lft forever
4: virbr0-nic: <BROADCAST,MULTICAST> mtu 1500 qdisc fq_codel master virbr0 state DOWN group default qlen 1000
link/ether 52:54:00:3c:0c:63 brd ff:ff:ff:ff:ff:ff
18: vnet0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel master virbr0 state UNKNOWN group default qlen 1000
link/ether fe:54:00:e7:18:69 brd ff:ff:ff:ff:ff:ff
inet6 fe80::fc54:ff:fee7:1869/64 scope link
valid_lft forever preferred_lft forever
19: enp0s20u7: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UNKNOWN group default qlen 1000
link/ether 0e:67:33:17:48:a0 brd ff:ff:ff:ff:ff:ff
inet 192.168.42.188/24 brd 192.168.42.255 scope global dynamic noprefixroute enp0s20u7
valid_lft 7175sec preferred_lft 7175sec
inet6 fe80::b0e:b959:2f6a:926e/64 scope link noprefixroute
valid_lft forever preferred_lft forever
enp0s20u7
is my phone's connection.