I get the 503 Backend fetch failed error when I try to connect to my Magento backend that is hosted on a AWS Lightsail Instance - 2 GB RAM, 1 vCPU, 60 GB SSD. I found the general tutorial on how to fix it https://devdocs.magento.com/guides/v2.3/config-guide/varnish/tshoot-varnish-503.html, but the path described there does not exist on my instance.
However, I found another tutorial https://community.bitnami.com/t/error-503-backend-fetch-failed-megenta-after-installation-extension/59593 that states it would also work to take the path /opt/bitnami/varnish/scripts/ctl.sh (that applied to my configuration), open the file and add to the VARNISH_OPTIONS parameter string i.e. "-p http_resp_hdr_len=65536". Still, I get the 503 Backend fetch failed error. This seems to be a common problem, I would appreciate any help with it!
I cannot even SSH into my console as the connectivity is so weak (or lost) that I cannot run the bitnami support tool.