Тогда надо посмотреть вывод команд ip r и ip a до ifup и после ifup.
[root@comp-celeron-cpu-d049ee ~]# ip r
default via 10.64.64.64 dev ppp0 proto static
10.64.64.64 dev ppp0 proto kernel scope link src 10.201.148.69
[root@comp-celeron-cpu-d049ee ~]# ifup ppp0
[root@comp-celeron-cpu-d049ee ~]# ip r
default via 10.64.64.64 dev ppp0 proto static
10.64.64.64 dev ppp0 proto kernel scope link src 10.201.148.69
[root@comp-celeron-cpu-d049ee ~]# ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 brd 127.255.255.255 scope host lo
valid_lft forever preferred_lft forever
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: enp5s0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc mq state DOWN qlen 1000
link/ether 00:1b:38:d0:49:ee brd ff:ff:ff:ff:ff:ff
3: wlp6s0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc mq state DOWN qlen 1000
link/ether 00:1f:3a:54:13:91 brd ff:ff:ff:ff:ff:ff
4: wwp0s29f7u1i1: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN qlen 1000
link/ether 02:50:f3:00:00:00 brd ff:ff:ff:ff:ff:ff
5: ppp0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN qlen 3
link/ppp
inet 10.201.148.69 peer 10.64.64.64/32 scope global ppp0
valid_lft forever preferred_lft forever