Can a Tech Lead externally mentor Scrum Team?

I'm not sure where the idea comes from that you are "not supposed to be needing help from outside the team". Cross-functional doesn't mean that the team can't turn to people outside the team to support their work. What it means is that the team should not constantly need people from outside the team in order to get work to done. There are plenty of times when the team may be learning new skills or have a need for very in-depth expertise that is not reasonable to expect to be fully embedded within the team or each of the teams within an organization.

There are a few ways to approach this, but assuming that the tech lead has the knowledge and ability to help teach the team techniques and practices and tools to make better architectural and design decisions, it makes sense to try to involve this person. The team should want to do this, though, rather than having it forced upon them - this is the self-organizing and self-managing aspects of a team. One possible path could be to talk to this tech lead to make sure that they are capable of providing the help that the team needs, and then bringing this up at a retrospective to see if it's a path the team wants to go down. If the team agrees, everyone can work together to determine how to best fit the tech lead's skills into the team on a temporary and/or part-time basis.

He says that he will not interfere with our work because we are supposed to be cross-functional and self organizing but since we have made some architectural missteps, I think it would be good if he helped us with some suggestions

He can suppose whatever he likes, but unless he knows a magic trick the reality is that you are not cross-functional and self-organizing. Moreover you will not become so until the appropriate leadership is evidenced, or until you have learned further expensive lessons.

Bear in mind that you are also supposed to meet a Definition of Done which satisfies organizational quality standards including architectural ones. Failure to do so will result in the technical debt you are now accruing. Dealing with technical debt costs the organization money, time, and reputation.

Agile leaders ought to exemplify servant leadership by clarifying Done standards and by providing useful advice and guidance. They should help teams reach the point where they can indeed meet these standards in a self-managing way. Those teams then become empowered to further improve upon them.

Thank you for sharing!

If your Tech Lead and Developers have no issues, I don't think its wrong to take guidance from your Tech Lead. This could be a stepping stone in making your Developers self reliant in future.

We have had cases where we had to take guidance from subject matter experts for tasks which involved use of technology which was very new to the Team.