My advice would be to get rid of any old Asus DSL modem/routers and never connect them to your phone line. They all have the notoriously bad Mediatek modem chipset known to have a load of problems with Openreach VDSL2 services. Poor speeds, dropping connections often resulting in DLM banding the line are likely to happen.
The list of bad devices includes the following :- N17U, AC52U, AC56U, N66U, AC68U
If you really want an ASUS device the DSL-AC88U has the much more acceptable Broadcom modem chipset and is generally considered a good choice, it is several years old so there may be newer models available for which I have no details, but don't get one with a Mediatek chipset.