This device?
https://www.router-reset.com/info/ECI/B-FOCuS-O-4G2PW
https://www.manualslib.com/manual/929057/Eci-Telecom-Ont-B-Focus-O-4f2pw.html?page=20#manual
I believe then, that you mean "router" - not "modem". However, modem and router can be separate or combined.
When you use "restart" do you mean that the power actually goes off (no status lights, etc.,) or that internet connectivity is momentarily lost?
Reference the Status LEDs shown on physically numbered Page 12 of the manual - which lights are on or off when the router restarts?
Either way, the symptoms suggest a loose connection somewhere. Electrical power or the ISP service connection to the modem.
Check all plugs and cables. Replace, where possible, existing cables with known working cables.
Could even be the electrical outlet itself or a problem in the circuit serving the wall outlet. Do any other devices experience problems: lights flicker, tv blips/goes briefly black, etc.?
If the restart problem continues then the modem, being 5 years old, may simply be starting to fail. Some component has degraded and for what ever reason just triggers a restart.
Do you own the modem/router or are you renting it from your ISP? If renting, then your ISP should be able to provide a replacement.
If you own the router then troubleshoot as best you can via the User Manual and cable swaps. Try another outlet if possible - just as a matter of elimination.