Here we have a sequence of three relative clauses, each of which begins with a direct object relative pronoun that refers to the antecedent the things:
do the things which I have told you [the things] that I have seen [the things] that your Lord and your Redeemer should do
In his editing for the 1837 edition, Joseph Smith deleted the second of these relative pronouns, thus ending up with “do the things which I have told you I have seen that your Lord and your Redeemer should do”. The resulting construction is more fluent, but the original reading works well enough. The critical text will restore the original that. For further examples of cases where a direct object relative pronoun such as which or that has been omitted, see under which in volume 3. For another example of where Joseph Smith edited a sequence of relative pronouns, see 1 Nephi 13:30.
Summary: Restore in 2 Nephi 31:17 the relative pronoun that which Joseph Smith deleted in his editing for the 1837 edition: “do the things which I have told you that I have seen that your Lord and your Redeemer should do”.