Getting a Huawei E3327 USB 4G modem to work with Raspberry Pi 4 running DietPi (Debian)

I want a Huawei E3327 to work as a fallback on a Raspberry Pi 4B. The modem shows constant cyan light, which means that there es a 4G connection established (I previously configured APN using a windows machine). I successfully used usb_modeswitch to switch the modem in the right state, and a a new network interface eth1 shows up. I configured a static IP, my /etc/network/interfaces looks like: # Drop-in configs source interfaces.d/* # Ethernet allow-hotplug eth0 iface eth0 inet dhcp address 192.168.0.100 netmask 255.255.255.0 gateway 192.168.0.1 #dns-nameservers 192.168.0.1 # WiFi allow-hotplug wlan0 iface wlan0 inet dhcp address 192.168.0.100 netmask 255.255.255.0 gateway 192.168.0.1 #dns-nameservers 192.168.0.1 wireless-power off wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf # hotspot iface wlan1 inet static address 192.168.42.1 netmask 255.255.255.0 gateway 192.168.42.1 dns-nameservers 192.168.42.1 #HUAWEI E3372 allow-hotplug eth1 iface eth1 inet static address 192.168.8.100 netmask 255.255.255.0 network 192.168.8.0 broadcast 192.168.8.255 There is also the internal WiFi connection of the RasPi and an external USB WiFi dongle configured as a hotspot, which works. The problem: I can only ping 8.8.8.8 when there is a wifi connection. The huawei modem does not seem to work. Without the wifi connection I get Network is unreachable when I try ping 8.8.8.8 with established wifi connection, ip address shows: 1: lo: 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 2: eth0: mtu 1500 qdisc mq state DOWN group default qlen 1000 link/ether d8:3a:dd:01:c0:55 brd ff:ff:ff:ff:ff:ff 3: wlan0: mtu 1500 qdisc pfifo_fast state UP group default qlen 1000 link/ether d8:3a:dd:01:c0:56 brd ff:ff:ff:ff:ff:ff inet 192.168.0.136/24 brd 192.168.0.255 scope global dynamic wlan0 valid_lft 603872sec preferred_lft 603872sec 4: wlan1: mtu 2312 qdisc mq state UP group default qlen 1000 link/ether 34:60:f9:64:4f:98 brd ff:ff:ff:ff:ff:ff 5: eth1: mtu 1500 qdisc pfifo_fast state UP group default qlen 1000 link/ether 00:1e:10:1f:00:00 brd ff:ff:ff:ff:ff:ff inet 192.168.8.100/24 brd 192.168.8.255 scope global eth1 valid_lft forever preferred_lft forever How can I make the modem work and act as a fallback when there is no WiFi connection?