> PEE151R4HEL.LCC8.2TFECE.EOT.901OTHN.ANT23P2RRAG.SNO0.O0TULE.

More gibberish!  At least this time it's not fake Chinese, but upper
case letters, numbers and periods.  Brief analysis suggests this is
encrypted text (writing it in allcaps is a classic) using
transposition (it's too short for frequency analysis and the index of
coincidence hints at English) which gives us a specific hint how to
proceed.  The numbers could be a port and IP address to connect to, so
they should line up to one if we're doing things right.  Most
suspiciously, the length of the ciphertext is 60, this allows
arranging it in rectangle shapes of 2x30, 3x20, 4x15, 5x12 and 6x10.
Trying all of them out, the 5x12 arrangement looks most promising as
each line ends in a period:


    PEE151R4HEL.
    LCC8.2TFECE.
    EOT.901OTHN.
    ANT23P2RRAG.
    SNO0.O0TULE.

If you read it each column down, it spells out the following:

> PLEASECONNECTTO18.205.93.120PORT1204FORTHETRUECHALLENGE.....

Alright, let's connect with `nc 18.205.93.120 1204`:

    YPWI5WORASODSZ772USR:CO
    ^C
    [...]
    YPWICYORASODSC07OUSR:LG
    ^C
    [...]
    YPWIHRORASODSSS4JUSR:H7
    foo
    Invalid password

The service expects a password, probably related to the preceding line
of gibberish.  After staring at it for a while, you can't help but
notice it includes all letters for the word "password", so it must be
another transposition cipher.  The colon staying in the same place
confirms that it's fully deterministic.  Trying out a bunch of these
on https://www.dcode.fr/ shows that it's a Railway cipher.  For
example the last line can be arranged into the following zigzag lines:

    YPWIHRORASODSSS4JUSR:H7

    Y   P   W   I   H   R
     O R A S O D S S S 4 J
      U   S   R   :   H   7

    YOURPASSWORDIS:SHSH4RJ7

Writing some code to do this for us isn't terribly hard and eventually
the prompt can be bypassed.  This is not the end though:

     #####
    ### #
     # #
    ######
    # ###
    ######

    PVEECR
    PO4LAE
    A4SNEE
    SSKWEO
    RNDMHT
    IOQRE:

At long last there's the Grille the challenge title alludes to.
Applying the mask to the text below gives you the first characters of
the plaintext:


     #####
    ### #
     # #
    ######
    # ###
    ######

    P
       L E
    A S EE

     N   T

    PLEASEENT[...]

To get the rest, rotate the mask and try again.  Like last time the
characters for "password" can be spotted in the ciphertext, so
presumably it contains another password:

    ### #
    # ####
    ### ##
    #### #
    ### ##
    # #  #

       E R
     O
       N
        E
       M
     O RE

    ERONEMORE[...]

    ######
     ### #
    ######
      # #
     # ###
    #####


    P   A

    SS W O
    R D
         :

    PASSWORD:[...]

    #  # #
    ## ###
    # ####
    ## # #
    #### #
     # ###

     VE C
      4
     4
      K E
        H
    I Q

    VEC44KEHIQ

Write some more code solving the cipher, submit the password and you
finally get a flag.