I've found out, that when I switch to static IP on interface ge00 and insert values:

uci set network.ge00.proto=static
uci set network.ge00.ipaddr=172.20.34.5
uci set network.ge00.netmask=255.255.255.224
uci set network.ge00.gateway=172.20.34.1
uci set network.ge00.dns=8.8.8.8

it doesn't make default route, so I have to do it manually like:

root@cerowrt:~# cat /etc/rc.local
#!/bin/sh
route add -net 0.0.0.0 netmask 0.0.0.0 dev ge00 gateway 172.20.34.1
exit 0