ok a 64bit system. Download following packages on a different computer. Store the files on an additional USB pen drive. You could store it on the HDD directly as well.
http://ftp.debian.org/debian/pool/main/i/ifupdown/ifupdown_0.8.36_arm64.deb
http://ftp.debian.org/debian/pool/main/i/iproute2/iproute2_5.10.0-4_arm64.deb
http://ftp.debian.org/debian/pool/main/i/isc-dhcp/isc-dhcp-client_4.4.1-2.3_arm64.deb
Once done connect the drive to the RPi4 and navigate into the directory you stored files on. Installation can be done as follow:
dpkg -i *.deb
Reboot the system. Usually this should get your network connection back.