First post and hoping someone can help me sort a long running problem.

I have a 2862ac running fw 3.9.8.1 which will randomly reboot at different times of the day. This has been an issue since Sep '21 with Draytek even replacing the internals. I have a VDSL+ line (1.4k long so slow speeds) from Zen that has been checked out along with the internal house wiring by OpenReach. While I can connect the DSL to the 2862 and get connected, the reboots are so frequent (20 hours uptime is good) the exchange does not like all these reboots and slows the line even more. Log files are sent (almost) daily to Tech Support - the ticket has been open since 6th Dec '21.

I am currently having a stable DSL connection via Fritz!Box 7530 ex Zen feeding to the WAN2 port and effectively only using the 2862 to provide home WiFi over 3 SSIDs. Both routers are connected to the same power outlet on a UPS and the home electricity supply appears stable. There are no VPNs, hardware acceleration switched off and two port forwarding rules.

I do have some 70 devices with fixed IPs being a mix of IP cameras and "smart" devices - not all of which are connected at any one time. The 2862 does not appear to be overheating - a "smart" thermometer on the top case shows max of 28C. CPU usage seldom goes above 7% with memory usually below 65% - there are only two people using the system most days. uPNP is off.

Most peripherals NAS, TV, cameras are connected via CAT6 cable via unmanaged switches (netgear or tp-link). I have two AP903 and a AP918R connected as a wired mesh - the mesh was created a few weeks ago,long after this problem materialised.

Prior to Sep '21, the router had been up for 1,986 hours so it would seem that it could do same. Can anyone suggest what I can look at to try and resolve this issue? I would like to be able to put the DSL into the 2862 and use WAN2 for an LTE input. Thanks for any help.