Unless a filing extension was requested, it's now too late to establish a SEP plan or a solo 401(k) plan to receive contributions for 2022.
If you did request a filing extension, you can establish either a SEP plan or a solo 401(k) plan, but generally not both. The contribution to a SEP plan is an employer contribution and you can make the same employer contribution to a solo 401(k) plan, although possibly limited if you make elective deferrals or Roth contributions to the solo 401(k) plan that you cannot make to a SEP plan. In other words, there is no tax benefit to establishing a SEP plan over a solo 401(k) plan because you can always contribute more the the solo 401(k) than to a SEP plan (unless you have no net profit, in which case you couldn't contribute to either).
Due to changes in the tax code, you have until the due date of your tax return to establish a 401(k) plan and make contributions. However, if you already maintained a SEP plan for 2022 based on Form 5305-SEP, you are not permitted to also establish a 401(k) plan for 2022.
