"Secure" Internet MQTT without TLS? Any ideas?

Heya!

I’m currently in a bit of a “dilemma” here… I’ve recently got an adapter for my Epever Solar Charger to connect a ESP8266 to it, however after some searching around I found that the ESP8266 doesn’t appear to support TLS via MQTT sadly due to hardware limitations.
I can’t create a VPN tunnel without having something like a Raspberry at that remote location due to power restrictions (fully off-grid with no big enough battery to run one 24/7), so I’d have to connect my two ESP8266’s there directly to HiveMQ (a cloud MQTT broker). However they require TLS to be enabled.

My idea was to simply host one myself, but I’m not entirely sure how to even make a secure TLS-less server if that’s even possible… so, do any of you have some ideas maybe?

Thanks already for suggestions!