If a network issue is interfering with connections to your site when accessed in plaintext HTTP, but not doing so when accessed via HTTPs, simply making sure that all your visitors access your site via HTTPs should be sufficient.
The currently only method to do that is become eligible and submit to the HSTS preload list and then wait for browser vendors to include your site. This process is not immediately effective, and only affects some browsers (but most of the currently relevant ones). After your domain name is shipped, all visitors will connect to your server securely, regardless of whether they are forwarded or manually typing the name.
This treats the symptom. To treat the actual problem, you will have to provide details about why that particular ISP is causing trouble:
- unintentionally breaking things by running broken software?
- wilfully interfering because government authorities told them so?
- wilfully interfering because a court order told them so?
The domain name, the ISP and the affected jurisdictions/regions would likely pinpoint the issue and help decide which resolution methods may be appropriate, even if the ISP is unresponsive/uncooperative.