How to Optimize This Code?

Post 10 above is actually the way to go with this problem...as it just references the required column and replaces the correct lookup found values...

The first is that it clears the following column to the right

This was just a solution for your sample file uploaded...One would need to use a helper column at end of data...

I can't run another macro that concatenates the values on column M and N

As far as above...One can only see if you upload a sample file to test...As we cannot recreate the issue

Just for future reference...the below declarations...
Only the red ones are actually declared...the ones prior without As are declared as variants...

Dim cel, fr As Range
Dim k, g, ofst, lr, frr As Long
Dim i, j As Long

Should be...

Dim cel As Range, fr As Range
Dim k As Long, g As Long, ofst As Long, lr As Long, frr As Long
Dim i As Long, j As Long