thanks for your request.
It's a bit hard to give some advice without seeing configurations and logs/ike trace in different cases.
My suggestion would be to please send an email to email@example.com with all the details, as:
-IMEI of the WR21
- debug.txt (https://www.digi.com/support/knowledge-base/how-to-extract-the-debug-txt-file-from-a-digi-tran) and IKE trace (https://ftp1.digi.com/support/documentation/QN_045_How_To_setup_analyser_To_Get_IKE_IPsec_trace.pdf) in all test cases (cable unplugged, cable plugged but fiber router not working, cable plugged and fiber router working).
With this email, a case will be opened and we can do a sanity check on the config and have a look at logs, but please also review our support options, as VPN troubleshooting is limited on Base level and would require an Expert contract if deep troubleshooting is needed.
Digi Technical Support