We need to start with a clarification.
An "employer" plan is not an IRA. Qualified employer plans might be a 401(k), a 403(b), a 401(a), a 457 plan, or something else. (The number refers to the section of the tax code that authorizes the plan.) An IRA is an individual account you set up with a bank or broker. Even though they have similar purposes to qualified workplace plans, they are covered by different sections of the tax law, and have some rules that are very different.
In particular, you generally can't contribute to a workplace plan once you leave the employment of the sponsor. You can contribute to a different account with the same trustee, but it is a different account. For example, if you have a 401(k) at Fidelity, and you leave that employer, you can probably open a traditional IRA at Fidelity, but that will be a private account separate from the 401(k) account, and you can't contribute directly to the 401(k) account.
Then, if you contribute to an IRA (no matter who the trustee is) you can make deductible or non-deductible contributions. Your ability to make deductible contributions may be limited based on your income and employment situation. You can also have a Roth IRA if you prefer, but that must be set up differently.
So now, before we can really help you, we need to know more details about what you are actually doing.
