John Gill Commentary Numbers 9:14

"And if a stranger shall sojourn among you, and will keep the passover unto Jehovah; according to the statute of the passover, and according to the ordinance thereof, so shall he do: ye shall have one statute, both for the sojourner, and for him that is born in the land." — Numbers 9:14 (ASV)

And if a stranger shall sojourn among you, and will keep the
passover unto the Lord
Then he must become a proselyte of righteousness, and be circumcised, or otherwise he might not eat of the passover, (Exodus 12:48) ; Ben Gersom interprets this of the second passover, and of a proselyte that was not obliged to the first, he not being then a proselyte, but became one between the first and the second; and so Aben Ezra understands it of a second passover, though he observes, that some say the first is meant:

according to the ordinance of the passover, and according to the
manner thereof, so shall he do ;
according to the several rites and ceremonies, whether of the first or second passover, that an Israelite was obliged to observe, the same a proselyte was to observe, and what they were has been already taken notice of:

you shall have one ordinance, both for the stranger and for him that
was born in the land :
for a proselyte, and a native of Israel; see (Exodus 12:49) .