VCU1525 has 2 x QSFP28 connectors, so you link 2 boards with 2 x 100 gigabit ethernet cables, and two boards is the max that can be daisy chained.  With the Bittware XUPP3R board, since it has 4 x QSFP28 connectors, there is no limit to the daisy chain length.  These specialized ethernet cables have nothing to do with internet, they are solely so data can flow from one FPGA board to the next board.  The cables are around $40 each.

Personally I think GPU mining will still be around for a while.  I believe the market is going to rise so dramatically this fall that all existing GPU rigs will be making a lot of money, even with rises in difficulty.