You might want to consider using Flexcel to do XLSX export.
Note there is no dedicated grid export component available to do this so it will most likely require a lot of extra coding.
More information can be found on the Flexcel product page and if you have any further questions about XLSX export you can post those in the Flexcel forum.
It won't be possible to use Flexcel in the same way as TIWAdvWebGridExcelIO but it might provide a workaround for features not supported there.
For specifics, please post your question in the Flexcel forum (where the responsible developer can answer in more detail) or you can also use the support form: https://www.tmssoftware.com/site/support_mail.asp