Reverse proxy unavailable after a while

hi
so i have a reverse proxy to HA setup as per the instructions
works perfectly
but
after a while, 2 days maybe, i can’t connect to my HA remotely.
I’ve checked my router remotely and it’s accessible and the port is open for connection to HA.
but, the nginx server is not replying
so
must be a timeout or something?
any suggestions please as to what 8 can check/amend