Make an appointment (to a Mexico or Canada US consulate) and get your visa stamp. And if by land, do remember to ask for a new I-94 when re-entering.
Your not getting I-94 with the I-797 is commonly caused by two reasons.
1. On I-129, you chose consulate notification so that no proof needed of your current valid status. You want US consulate to examine your last stay in US, not USCIS.
2. You did want USCIS to check your current valid status since last entry, but you failed to prove that. So I-94 is not issued.
Your not getting I-94 with the I-797 is commonly caused by two reasons.
1. On I-129, you chose consulate notification so that no proof needed of your current valid status. You want US consulate to examine your last stay in US, not USCIS.
2. You did want USCIS to check your current valid status since last entry, but you failed to prove that. So I-94 is not issued.