For some reason the SNR does seem to fall on lines like ours. Interestingly whilst others had connection problems with the recent stormy weather I recorded a record SNR margin of 19dB. Of course within minutes it was falling to 17, 16, 12, 11
Often you can create specific exception rules through the web interface - and these controls should be obvious and often have their own tab/page.
Services in particular are often very easy to get running with a tick in a box and IP address.
Regular inbound traffic is not blocked by default, depending on the rules in use. Most traffic is a response outbound traffic. It is only normally unsolicited inbound traffic that is blocked by default.
Through the interface you should be able to set up port triggering/forwarding if required (e.g. web services) and enable things like uPnP (if you have this on your computer it can autonegotiate ports etc), ping response etc.