To manipulate TTL, you use an action=change-ttl rule in /ip firewall mangle.
The own traffic of the router should leave with TTL=255, but even that may not always be the case, so you need a mangle rule in chain postrouting which handles both the router's own traffic (such as DNS) and the forwarded traffic from LAN.
/ip firewall mangle add chain=postrouting action=change-ttl new-ttl=set:255 ttl=greater-than:200 out-interface=your-lte-interface-name
In order that it worked, you must not use fasttracking (so disable the action=fasttrack-connection rule in /ip firewall filter if there is one) because one of the reasons why fasttracking speeds up packet processing is that it skips mangle rules.
If you happen to already have any other mangle rules in chain postrouting, first issue a command /ip firewall mangle print chain=postrouting where !dynamic, and only then issue the command above with additional parameters passthrough=yes place-before=0.
Whether this will help or not is a question, as the mobile operator may also check the IMEI of the equipment, not just the TTL, an even not accept connection to particular LTE bands for "wrong" IMEIs.
The additional condition (ttl=greater-than:200) is there to allow traceroute to work (using the hotspot data quota probably). If you're never going to use traceroute, you may omit this condition.
Last edited by sindy
on Thu Jan 14, 2021 8:09 am, edited 1 time in total.
Instead of writing novels, post /export hide-sensitive. Use find&replace in your favourite text editor to systematically replace all occurrences of each public IP address potentially identifying you by a distinctive pattern such as my.public.ip.1.