Because the number can be up to 15 digits, you’ll need to cast to an 64 bit (8-byte) integer. Try this:
SELECT * FROM table
WHERE myint = mytext::int8
The :: cast operator is historical but convenient. Postgres also conforms to the SQL standard syntax
myint = cast ( mytext as int8)
If you have literal text you want to compare with an int, cast the int to text:
SELECT * FROM table
WHERE myint::varchar(255) = mytext