1. as you say you have tried different SIMs, were these from the same operator or from different operators?
2. as @gabacho4 suggests, it may actually not be an LTE issue but some weird behaviour of the ISP. So when "internet doesn't work", what does /interface lte info 0 show? And when it is in that state, do also the following:
/ip dns set servers="" (to use the DNS servers indicated by the ISP)
:put [resolve finetwork.es]
What is the output of the last command?
I think I have been misunderstood. I don't believe the issue is with the ISP. I have previously used an ASUS 4G router for months without any issues whatsoever and it was locked on to the same two channels (B1 and B20). I don't recall if I had it pointed at the ISP DNS or not but at this point, that is irrelevant. Despite having my LtAP LTE6 set to use my ISP DNS, I had another outage while using the aggregation mode. (B1 and B20). Running the commands suggested yields the following:
/interface lte info 0
# sep/25/2021 14:56:17 by RouterOS 6.48.4
# software id = DTC8-QA35
#
pin-status: ok
registration-status: registered
functionality: full
manufacturer: "MikroTik"
model: "R11e-LTE6"
revision: R11e-LTE6_V028
current-operator: 41902
lac: 41061
current-cellid: 395817
enb-id: 1546
sector-id: 41
phy-cellid: 368
access-technology: LTE (CA2)
session-uptime: 6h13m4s
imei: myIMEI
imsi: myIMSI
uicc: mySerial
primary-band: B1@20Mhz earfcn: 301 phy-cellid: 368
ca-band: B20@10Mhz earfcn: 6200 phy-cellid: 368
rssi: -47dBm
rsrp: -50dBm
rsrq: -8dB
sinr: 26dB
cqi: 15
ri: 2
I already had my DNS set to use the ISP but I double checked just to be sure:
/ip dns print
# sep/25/2021 14:57: 6 by RouterOS 6.48.4
# software id = DTC8-QA35
#
servers:
dynamic-servers: 212.43.18.22,95.66.18.22
use-doh-server:
verify-doh-cert: no
allow-remote-requests: yes
max-udp-packet-size: 4096
query-server-timeout: 2s
query-total-timeout: 10s
max-concurrent-queries: 100
max-concurrent-tcp-sessions: 20
cache-size: 2048KiB
cache-max-ttl: 1w
cache-used: 30KiB
lastly running :put [resolve google.com or 8.8.8.8] resulted in the following.
[ngoehring@MikroTik] > :put [resolve 8.8.8.8]
failure: dns server failure
[ngoehring@MikroTik] > :put [resolve google.com]
failure: dns server failure
after resetting the modem by setting the LTE to only use B1, things began to work again.
So here are my thoughts and, frankly, complaints:
1. ASUS router NEVER exhibited these behaviors despite connecting to the same channels. It was located in the same location as the LtAP.
2. Telling the LtAP to use ISP DNS makes no difference over using Google or some other provide DNS.
3. I NEVER have an issue with the LtAP if I have it set to connect to only one channel (B1). I had been running for months with no issues since I made that setting adjustment. After seeing this thread and thinking that perhaps V28 firmware would correct things, I decided to try to revert to the CA configuration. FAIL. This is very clearly a Mikrotik issue given bullets 1 and 3.