Dietpi fail to find eithernet connection at boot time

puuh this means you don’t have ip command as well as ifup?? This is more than bad as you will not be able to bring up the network interface without…

Still the question. Do you have a backup? As well can you check installation status of ifupdown package

dpkg -l ifupdown

It should looks like this

root@DietPi3:~# dpkg -l ifupdown
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name           Version      Architecture Description
+++-==============-============-============-================================================
ii  ifupdown       0.8.35       armhf        high level tools to configure network interfaces
root@DietPi3:~#

Looks like you hit the same situation like this user https://dietpi.com/forum/t/removal-of-pi-hole-removed-ip-command-stuck-in-boot-net-wait/4976/1