No internet after LB2120 switch back to wireline broadband

Upon power lost in the neighborhood, my wireline broadband will go out as well.

LB2120 will failover to 4G LTE.

I have internet access again.

Upon the return of wireline broadband, LB2120 will switch back to it.

However, my computer cannot connect to the internet anymore unless I cycle power the LB2120.

If I took the wireline broadband (input to LB2120) and plug it into my router (where output of LB2120 goes), my computer can connect to the internet.

This only happened on real lost of wireline broadband as described above.

I cannot reproduce this problem by just unplugging the wireline broadband from LB2120  and plug it back in a few minutes later after failover happened.

Dont know if it requires failover to 4G LTE for 3-10 hours like that in real life in order to repro.

Anyone has any idea what is wrong?

Any help would be appreciated.