Can you recommend a smart and reliable router with USB 3G/4G, WiFi 2.4 and 5, DynDNS?

Question

WiFi

Friends, I ask for help with the reorganization of external and internal routing.
Current situation: two rooms, office and warehouse, IP telephony, Windows Server 2008 R2 Enterprise server with 15 remote desktops, printers; Internet provider allocated a static IP; all routing - Asus RT-N16 on DD-WRT, 12 wired connections and a bunch of wireless ones (laptops, smartphones). For the convenience of users, masquerading is configured, they always connect from the internal network and from the outside using the same IP. There is no internal domain, just a regular locale.
Problems:
1) Asus started to suffer from garbage, after reboot it resets the settings, constantly feeding it a backup file is not an option
2) the provider has problems, and there is no alternative normal provider on our territory.
I am looking, firstly, for a reliable device (or a bunch of devices) that can receive the Internet both in the usual way and via a USB 4G modem and automatically switch to the mobile Internet in case of loss of connection with the main provider's gate, and secondly, a solution with connection from inside and outside for RDP. The idea with a VPN is understandable, but users will have difficulties with this for sure. What else could there be besides VPN and Dynamic DNS? If nothing else, then DynDNS or NO-IP must be supported on the router.

Answer 1

My ASUS RT-AC68U seems to be able to do everything you need

Answer 2

Zyxel Keenetic Extra

Answer 3

?

Answer 4

Asus RT-N65U with firmware from Padavan. Or Mikrotik. And no zyxels. Their firmware is a permanent beta.
With Wi-Fi 5 GHz, the problem will be on any device. Penetrating power is extremely low. A couple of walls - and no signal.