Post-upgrade problem (double nat and modem)

Hi, I finally decided to upgrade, I was still on the old v18, but some programs were starting to fail so I took the plunge.

First congratulations, it seems very responsive to me and the wifi seems to have better coverage in the house!

Obviously everything was working somehow before, now I have to fix some things, I will make more posts because they are about different problems.

I own an R7800 and before upgrading I saved the list of old packages and backed up the router, via the dedicated page.
Immediately I upgraded while keeping the configurations, but it took only half an hour of testing to make me reconsider, so I reset the router with the physical button (hold 10 seconds). I hope this operation is like resetting from the router interface
Once I had access to the router I started configuring it exactly as it was configured before the upgrade ie:
Internet -> fiber modem ( with DHCP) -> router (wan port -> my local lan (via DHCP
I have not checked if I can use PPPoE protocol but as I said it is the same configuration I had with v18 version

So far I have installed DDNS, UPnP and Simple AdBlock.
DDNS works without problems, while UPnP and Simple AdBlock "I think" create some problems.

The very first error I have now is during SSH connection via Putty, this error appears
-ash: fgrep: not found, I thought fgrep was part of grep which is installed.

The main problem now is that I can't reach the modem at the address, I tried to look for some solution, I found who was adding a modem peripheral with static address etc., but in the configuration before I didn't have anything like that, so I haven't changed anything yet

I created a report with these commands, so maybe you can figure out where I'm doing wrong

ubus call system board; uci show network; uci show firewall; \
ip address show; ip route show table all; ip rule show; iptables-save -c

Question, if I had the option, would it be better to bypass the double nat?

Thank you and sorry if I am not very knowledgeable on the topic of nat and network redirections in general

never mind, read it incorrectly.