Has anyone tried a Bluetooth to TTL adaptor for uart access?
I'm just looking for something I can install permanently and not have to dismantle the router and plug it in.
Pull the serial wires through some vent in the casing ?
Provided the signal voltages are correct and there is sufficient power for the module to run, I don't see any specific reason that it wouldn't work in theory.
I would warn that there could be a security risk here (short range, physically speaking, but real) in that these BT serial adapters rarely have any passwords or other authentication requirements -- just connect and go. This would mean that anyone could gain serial access to your device if they're within range.
But, if that isn't an issue for your situation, I'd say go ahead and try one... report back with your findings.
or... as @frollic suggested, use the simple approach to a wired connection.
I have seen solutions, but it seems they are all 3.3v or 5v sadly nothing 1.8v.
Some do have Pairing buttons and passcodes I did find this one that has a AT command set to change the pairing code and other features to power down the Bluetooth etc...
I might try it out i will just need to buy a logic level shifter
SH-HC-08 Bluetooth Module with 6PIN Base Board (deshide.com)
Yes, I use those extensively. Extremely useful for outdoor mounted devices for example: https://openwrt.org/toh/zyxel/nr7101#serial
I have been using "jdy-31" modules without any carrier board. They are running a slave only firmware which makes the configuration impler. And dropping the unnecessary carrier board avoids the regulator voltage drop. So far I have been using these as BR devices, but the last batch I got had a firmware with BLE support as well. Which I guess can be useful in some settings.
Personally I simply use rfcomm on a console server with a bluetooth dongle to manage these. The simplest one I got is a Unifi UAP AC Pro running OpenWrt with conserver 
The Bluetooth modules are configured by AT commands on their UART port. This management interface is active as long as there is no active Bluetooth connection. Definitely easiest to preconfigure before installation using a USB TTL adapter. But it is possible to configured them after mounting too, as long as you can access the attached device through some other channel to send commands out on the console port. Mostly useful for simple reconfiguration of device name etc.
The only issue I've had so far after installing these all over the place was a Netgear GS108Tv3 switch where the 3.3V terminal wasn't connected. Other than that, I've had great success with assorted routers, switches and APs.
I bought a new batch from this seller just a couple of months ago: https://www.ebay.com/itm/385916963172
Can definitely recommend both modules and seller.
EDIT: Example permanent (soldered!) connection to my brand new U6+. This has been nicely hidden inside the case on the now wall mounted AP, without anything visible on the outside:
EDIT2:
I've never tried this, but the jdy-31 or similar without any additional regulator might work on 1.8V. Provided that you have a 1.8V supply. The spec I got is a little sketchy, but it claims
Operating voltage: 1.8-3.6V (3.3V recommended)
I assume the IO voltages will be slightly below Vcc . You should probably veryfy this with a multimeter before frying anything.
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Definitely something to think about. The Bluetooth modules provide no security whatsoever. I enable "ttylogin" on the OpenWrt devices where I use these. But that's of course only a thin layer of protection. And anyone can still snoop console messages or interact with the bootloader.....
Good enough for me, given the close range which is required (less than 10 meters in my experience if there's a wall inbetween). But not something for the most hostile environments.
Devices like the ZyXEL NR7101 or Ubnt U6+ and U6 Lite have no vents.
But the real issue for me would be where to route the wires outside the casing. I want permanent console access so that I can log crashes or unexpected messages. Or simply run "console nr7101" when I want to test something. Without having to go outdoors and find a ladder to attach a console cable 
I have seen a post from someone using an ESP8266 or ESP32 to connect to the serial console through WiFi.
I'm working on this right now using an HC-05 - haven't had any luck successfully communicating with my router. Tested the bluetooth (works, put it in commandmode to set the baud rate and stop bit), am able to communicate with the bluetooth device over serial wired, am also able to communicate with the router over serial wired.
I've run the wires with dupont pins out of the vents on mine for the time being. The HC-05 needs 5v to work correctly, I believe ... so I'm pulling it off a USB plug as the router only puts out 3.3v on the serial bus.
Eager to know if you go it to work at all.
This guy used an HC-08, I believe.