You confusing an individual 401(k) with an IRA. An individual 401(k) is not an IRA, it's a different kind of qualified retirement account. You are also confusing a 401(k) and SEP-IRA with a regular personal IRA which is what Schwab is indicating has a $5,500 contribution limit for 2018.
The deadline for establishing an individual 401(k) to receive contributions for 2018 is December 31, 2018. It's likely too late to get an individual 401(k) established by this deadline.
