Best way to expose SSH service behind CGNAT?

I have a remote server running 24x7 which as a normal DSL connection over a router.

As a backup, I plugged in a Huawei LTE stick. Of course, the LTE connection is CGNAT, so I cannot open any services. What is the best and most reliable way to access my server in case the DSL connection is down that, if possible, does not need another server under my control?

The most basic options require another external server; then an SSH connection could be built to that server. Or an openvpn connection.

Another thing I thought of is a tor hidden service. But I'm not sure if this isn't overkill.

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Any creative advice?