I believe that if you sign up for a RealVNC account (free), you will be able to remotely access the Pi without needing to change anything on the router. I haven't tried it myself, but I'm sure details are available on their website.
Thanks to Westie for the prompt on RealVNC cloud usage:
While still mourning the loss of MDWS I have been investigating other options to keep any eye on my son's & daughter's VDSL2 lines remotely having invested in replacement (stat's friendly) routers and RPi Zero W's for both last year, my own line not being such a problem as monitored locally by an old RPi Mk.1B all now running Eric's latest DSLStats V.6.3.
As Westie suggested, set up a free RealVNC account and was surprised just how easy it was to set up the 3 RPi's for remote, cloud access, I still run Raspbian Jessie on the RPi's and VNC Server is fully integrated by default into this system, was already operational for LAN/WiFi local access to manage the respective Pi's, software/program updates etc. when required on the old DSLStats/MDWS set up.
Local access to each RPi is now available either by LAN/WiFi or cloud, an option to still permit local access as well as cloud access is available during the set up procedure, I chose both.
Obviously a poor substitute for MDWS with it's superb data recording, trending and archiving facilities but in my case a partial solution and justification to keep the two remote RPi's operational running DSLStats and will enable me to keep an eye on the families lines performance, with Eric's recent changes to the recording page time scale options on DSLStats Rev.6.3, records archiving space can now be greatly reduced and is easier managed.
Again, many thanks to Eric (Roseway) for so promptly engineering the changes (to date) to DSLStats post MDWS.
Edit:
No changes to any of the routers configurations were required.
Local router as per footer, remote routers, Billion 8800NL r1 and r2.